A hidden, self-replicating software program, usually containing malicious logic that propagates by inserting copies of itself into and becoming part of another host program. It is designed to infect and gain control over vulnerable systems without the user's knowledge or consent and is activated when a user runs or opens its host program. Computer viruses can cause frequent computer crashes or pop-up messages, corrupt or delete data on a computer, reformat the hard drive, use an email program to spread the virus to other computers, or flood a network with traffic, ultimately making it impossible to perform any Internet activity.

Software that is designed to infiltrate and damage a computer system without the computer owner's knowledge or consent. The term is a shortened combination of the words "malicious" and "software" and is a generic term used to refer to a variety of forms of hostile or intrusive software, such as computer viruses, worms, Trojan horses, and spyware.

A generic term typically describing software that records, without the knowledge of the computer owner, information about the user, including browser habits or other information, and then transmits that information across the Internet. Spyware 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. Spyware is often associated with software that displays advertisements or software that tracks personal or sensitive information.

A type of malicious software (malware) named after the wooden horse the Greeks used to infiltrate Troy that masquerades as a legitimate computer program such as a game, image file, disk utility, or even an antivirus program. Users are typically tricked into downloading Trojan horses on their systems because they appear in the form of benign, useful software or files from a legitimate source. Once activated, Trojan horses can perform a number of attacks on a computer system that can cause pop-up windows to delete files, steal data, give malicious users access to a system, or activate and spread other malware, such as viruses. Unlike computer viruses and worms, a Trojan horse does not reproduce by infecting other files; nor do they self-replicate.