Malware refers generally to any malicious software program. Malware is often designed to run without a user's knowledge or approval.
There are many forms of malware, and new forms are constantly being created. In many cases, a particular piece of malware may fall into more than one category, for example adware that also functions as spyware. Further, any given malware threat may include a combination of different components.
Adware is any software program that displays advertising banners. Some adware may also be spyware, if it includes code that tracks a user's personal information.
A backdoor is a tool that an attacker installs to gain access to a compromised system.
A Bot is a piece of software that performs an automated task over a network. Harmful bots include those that allow a remote user to control your computer, or that scan your hard disk in order to steal your personal information.
A Botnet is a group of bot-infected computers that are controlled remotely. Botnets can be coordinated to attack a particular network or system, or to generate enormous quantities of spam email.
A browser hijacker is a program designed to alter a computer user's browser settings. These can replace a user's home page, or redirect the browser to malicious URLs when certain sites are visited.
A dialer is a program that replaces the phone number in a modem's dial-up connection. Dialers operate without the user's knowledge. They can send keylogging or other information to a third party, or they can be used to generate long-distance or pay-per-call charges.
File-sharing programs transfer files from one computer to another over the Internet using a peer-to-peer connection. File-sharing itself is not inherently harmful, but in some cases commercial file-sharing clients may contain adware or spyware.
A keylogger (keystroke logger) is software that records a user's keystrokes in order to steal passwords or other information. Keyloggers can be distributed as viruses or worms, or they may make up the payload of a trojan.
Phishing attempts to steal personal information by masquerading as a message from a trusted source, such as a bank or other institution. Phishing often relies on convincing a user to enter their personal or financial information at a particular web site.
A rootkit is a collection of tools (programs) that are used to hide the presence of malware (and the rootkit) on an infected computer.
Spyware is any software that collects and sends information without a user's knowledge. This may include tracking your browsing history or stealing credit card numbers or other information.
A Trojan is a piece of software that delivers a malicious payload (such as a virus or worm) while appearing harmless. Unlike viruses, trojans cannot reproduce by themselves; each new victim must run the trojan in order for it to deliver its payload.
A virus is software that reproduces itself and infects files without the user's knowledge or approval. In the simplest case, viruses spread to other computers when the file they infect is physically transferred via a disk, USB drive, or other media. They can also spread over networks when infected files are sent from one system to another.
Some viruses do nothing but replicate themselves over and over, which may lead to system instability and crashes. Other viruses are designed to directly damage files on the host computer.
A worm is a program that can reproduce itself without the need to infect files. It uses the network to send copies of itself to other systems and can spread extremely rapidly. Worms are specifically designed to harm networks, if only by using up network resources.