Adware
Adware is software which automatically plays, displays, or downloads advertising material to a computer after the software is installed on it or while the application is being used.

Keylogger
Imagine that you have typed your credit card details into an online order form for a new digital camera or plasma television, which I’m sure you have done for many products – we all have. Without your knowledge, a small program is inadvertently downloaded on to your machine while surfing or as an executed email attachment, lurking within your computer system capturing every keystroke you make; even capturing an image of your screen. These details are then surreptitiously emailed, not using your own email client of course, to a third-party (the cybercriminals out to fleece you for every penny you have). The first time you will know about the fraud is when your statement arrives showing withdrawals you know you did not make. Very creepy and easy to accomplish. Kaspersky, a leading anti-virus developer, reported a 500 per cent increase in recorded leylogger infections between January 2003 and July 2006 and the figure continues to rise. There are about 300 families of keyloggers waiting to catch their next victim.

Malware
During the 1980s and 1990s, it was usually taken for granted that malicious programs were created as a form of vandalism or prank. More recently, the greater share of malware programs have been written with a financial or profit motive in mind. This can be taken as the malware authors' choice to monetise their control over infected systems: to turn that control into a source of revenue, e.g., spambots. In this context, as throughout, it should be borne in mind that the “system” under attack may be of various types, e.g. a single computer and operating system, a network or an application. It is not only operating systems that need to be updated to fix exploits but applications too. As Microsoft’s Windows operating system team has become proactive in reducing the number of exploits that exist, Microsoft Office and other applications are becoming the target to exploit. The message is to update everything regularly. Malware ("malicious" and "software) is software designed to infiltrate or damage a computer system without the owner's informed consent. The term malware is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code. Software is considered malware based on the perceived intent of the creator rather than any particular features. It includes computer viruses, worms (network-borne infectious programs), Trojan horses, spyware, dishonest adware, ransomware and rootkits and other malicious and unwanted software which eclipses all types of infections previously in existence. Since the rise of widespread broadband Internet access, more malicious software has been designed for a profit motive. For instance, since 2003, the majority of widespread viruses and worms have been designed to take control of users' computers for black-market exploitation. Infected "zombie” computers (or “zombies” for short) are used to send email spam (machines infected are called spambots), to host contraband data such as child pornography, or to engage in distributed denial-of-service attacks as a form of extortion against companies and software developers fighting malware. Another strictly for-profit category of malware has emerged in spyware - programs designed to monitor users' web browsing, display unsolicited advertisements, or redirect affiliate marketing revenues to the spyware creator. Spyware programs do not spread like viruses; they are generally installed by exploiting security holes or are packaged with user-installed software. The best-known types of malware viruses and worms, are known for the manner in which they spread, rather than any other particular behaviour. Originally, the term computer virus was used for a program which infected other executable software, while a worm transmitted itself over a network to infect other computers. More recently, the words are often used interchangeably. Today, some draw the distinction between viruses and worms by saying that a virus requires user intervention to spread, whereas a worm spreads automatically. Using this distinction, infections transmitted by email or Microsoft Word documents, which rely on the recipient opening a file to infect the system, would be classified as viruses, not worms. Most types of infections are written for the Microsoft Windows operating system and in particular the Windows NT-based operating systems such as Window 2000, Windows XP and eventually Windows Vista. Concealment can also help install malware. By disguising a malicious program as something innocuous or desirable, users may be tempted to install it without knowing what it does. This is the technique of the Trojan horse. Often attempting to delete malicious software on a computer may activate the software, causing damage and the infection of other files in the computer. Broadly speaking, a Trojan horse is any program that invites the user to run it, but conceals a harmful or malicious payload. The payload may take effect immediately and can lead to many undesirable effects, such as deleting all the user's files, or more commonly it may install further harmful software into the user's system to serve the creator's longer-term goals. Trojan horses known as droppers are used to start off a worm outbreak, by injecting the worm into users' local networks. One of the most common ways that spyware is distributed is as a Trojan horse, bundled with a piece of desirable software that the user downloads off the Internet or a peer-to-peer file-sharing network. When the software is installed, the spyware is installed alongside. Spyware authors who attempt to act legally may include an end-user license agreement which states the behaviour of the spyware in loose terms, but with the knowledge that users are unlikely to read or understand it. Once a malicious program is installed on a system, it is often useful to the creator if it stays concealed. Techniques known as rootkits allow this concealment, by modifying the host operating system so that the malware is hidden from the user. Rootkits can prevent a malicious process from being visible in the system's list of processes, or keep its files from being read. (See: Rootkit (or Root Kit)). A backdoor is a method of bypassing normal authentication procedures. Once a system has been compromised (by one of the above methods, or in some other way), one or more backdoors may be installed, in order to allow the attacker access in the future. Crackers typically use backdoors to secure remote access to a computer, while attempting to remain hidden from casual inspection. To install backdoors crackers may use Trojan horses, worms, or other methods. Another way that financially-motivated malware creators can monetise their infections is to directly use the infected computers to do work for the creator. Spammer viruses, such as the Sobig and Mydoom virus families, are commissioned by e-mail spam gangs. The infected computers are used as proxies to send out spam messages. The advantage to spammers of using infected computers is that they are available in large supply (due to virus infection) and they provide anonymity, protecting the spammer from prosecution. Spammers have also used infected PCs to target anti-spam organizations with distributed denial-of-service attacks or extort money from companies with distributed denial-of-service attacks also. In order to coordinate the activity of many infected computers, attackers have used coordinating systems known as botnets. In a botnet, the malware or malbot logs in to an Internet Relay Chat (IRC: an IRC bot runs hidden, and complies with the RFC 1459 standard) channel or other chat system or server. The attacker can then give instructions to all the infected systems simultaneously. Botnets can also be used to push upgraded malware to the infected systems, keeping them resistant to anti-virus software or other security measures. Lastly, it is possible for a malware creator to profit by simply stealing from the person whose computer is infected. Some malware programs install a keylogger, which copies down the user's keystrokes when entering a password, credit card number, or other information that may be useful to the creator. This is then transmitted to the malware creator automatically, enabling credit card fraud and other theft such as from EBAY and PayPal customers. Similarly, malware may copy the CD key or password for online games, EBAY and PayPal, allowing the creator to steal accounts or virtual items.

Ransomware
Since May 2005 malware extortion attacks (that encrypt plaintext data or delete data) have been appearing in greater numbers. Ransomware: cryptovirus, cryptotrojan or cryptoworm is a type of malware that contains and uses the public key of the extortionist to hybrid encrypting plaintext data belonging to the computer user, and demanding a ransom for its restoration using the private key (which is not in the malware) to decrypt the data back to plaintext. This type of ransom attack can be accomplished by, for example, attaching a specially crafted file/program to an e-mail message and sending this to the victim. If the victim opens/executes the attachment, the program uses the public key from the extortionist to hybrid encrypt the targeted plaintext data. A ransom note is left behind. The victim will be unable to open the encrypted files without the private key of the extortionist. Once the ransom demanded is paid, the data may or may not be decrypted back into plaintext (this depends on the so-called goodwill of the extortionist). Not a nice position to be in if the “kidnapped” data is important and no backup is available.

Rootkit
Originally (dates back to the days of Unix), and in some cases today it is used within the legitimate software development domain. The purpose of rootkit technology was not always used to attack and gain control of a computer but in benign applications. Therefore, rootkit technology may be used for both productive and destructive purposes. The term rootkit (also written as Root Kit) is a general description of a set (or suite) of programs which work to subvert (circumvent) control (to take full control) of an operating system from its legitimate operators or open security holes that allow viruses to break in causing other kinds of havoc by modifying many of the tools and libraries, which programs upon system depend. Usually, a rootkit will obscure its installation and attempt to prevent its removal through a subversion of standard system security. Techniques used to accomplish this can include concealing running processes, network connections, memory addresses, registry entries, files, or system data from the operating system and other programs used by system administrators to detect intended or unintended special privilege accesses to the computer resources. Recently, rootkits have been used increasingly by malware to help intruders maintain access to systems while avoiding detection. A rootkit is often used to hide utilities. These are often used to abuse a compromised system, and often include so-called "backdoors" (in the form of hard-coded username/password pairs, hidden command-line switches or magic environment variable setting) to help the attacker subsequently access the system more easily. For example, the rootkit may hide an application that spawns a shell when the attacker connects to a particular network port on the system. Kernel rootkits may include similar functionality by adding additional code and/or replace a portion of the kernel code with modified code to help hide a backdoor, for example. A backdoor may also allow processes started by a non-privileged user to execute functions normally reserved for the super-user. All sorts of other tools useful for abuse can be hidden using rootkits. This includes tools for further attacks against computer systems which the compromised system communicates with, such as sniffers and keyloggers. A possible abuse is to use a compromised computer as a staging ground for further abuse (a zombie computer). This is often done to make the abuse appear to originate from the compromised system or network instead of the attacker. Tools for this can include denial-of-service (DDoS or DoS) attack tools, tools to relay chat sessions, and e-mail spam attacks. A major use for rootkits is allowing the programmer of the rootkit to see and access user names and log-in information for sites that require them. The programmer of the rootkit can store unique sets of log-in information from many different computers. This makes the rootkits extremely hazardous, as it allows Trojans horses to access this personal information while the rootkit covers it up. Rootkit technology first came to the attention of the general computer using public in October 2005, by Windows expert Mark Russinovich who broke the news about a truly underhanded CD copy-protection technology by Sony Music. The rootkit that Sony Music used to protect their CDs from being ripped, destabilised the computer it shoehorned itself into, and was very quickly used maliciously by malware creators.

Spyware
Spyware is computer software that is installed surreptitiously, most commonly distributed by a Trojan horse, on a computer system to intercept or take partial control over the computer user's interaction with the computer, without the computer user's informed consent. The functions of spyware extend well beyond simple spy monitoring. Spyware programs can collect various types of personal information, but can also interfere with user control of the computer in other ways, such as installing additional software, redirecting Web browser activity, or diverting advertising revenue to a third party. Running anti-malware software has become a widely recognised element of computer security best practices for Microsoft Windows operating systems, as it has become one of the pre-eminent security threats. Like recent viruses, spyware - by design - exploits infected computer systems (operating system or applications) for commercial gain. Typical tactics furthering this goal include delivery of unsolicited pop-up advertisements; theft of personal information (including financial information such as credit card numbers); monitoring of Web-browsing activity for marketing purposes; routing of HTTP requests to advertising sites (browser home page redirection), drive-by downloads using exploits from Microsoft Windows operating systems, Internet Explorer and Sun Java, to name a few. A spyware program is rarely alone on a computer. An affected machine can rapidly be infected by many other components, resulting in unwanted behaviour and degradation of system and network performance and disable software firewalls, anti-virus software and reducing security browser settings (in turn increasing the likelihood of further infections). Identity fraud, the cost to the individual directly (loss of funds) or indirectly (time lost convincing the agency involved that credit card purchases were the result of identity fraud) is now widespread.

Trojan
A Trojan horse is a malicious program that is disguised as or embedded within legitimate software.

Worm
A computer worm is a self-replicating computer program using a network to send copies of itself to other nodes (computer terminals on the network) and it may do so without any user intervention. Unlike a virus, the worm does not need to attach itself to an existing program. Worms always harm the network (if only by consuming bandwidth), whereas viruses always infect or corrupt files on a targeted computer. Worms primarily spread by exploiting vulnerabilities in operating systems or applications, or by tricking users to assist them. Initially worms were created to spread only. Nowadays, worms commonly include a "payload" such as with the Mydoom worm. A "payload" is code designed to do more than spread the worm, e.g., delete the host’s computer files, ransomware, or create a backdoor. A very common payload for worms e.g., Sobig and Mydoom, is to install a backdoor in the infected computer to allow the creation of a "zombie” computer, (or “zombie” for short) under control of the worm author. Networks of such machines are often referred to as botnets and are very commonly used by spam senders for sending junk email, called spambots. Backdoors, however whatever the vector of infection is, can be exploited by other malware infections, including worms. Once a system is infected, varying multiple infections may ensue increasing the likelihood of further malicious infections.

Virus
A program that can copy itself and infect a computer without permission or knowledge of the user.