Viruses are self-executing, self-replicating programs. They alter the way a computer operates without the knowledge or permission of the user. When activated, viruses may damage files, cause erratic system behavior, or display annoying messages. The ability to self-replicate differentiates viruses from Trojan horses, worms, and other virus-like programs.
Worm is a self-replicating virus that does not alter files but resides in active memory and duplicates itself. Worms use parts of an operating system that are automatic and usually invisible to the user. It is common for worms to be noticed only when their uncontrolled replication consumes system resources, slowing or halting other tasks.
Trojan horses are malicious programs disguised as something benign. They've been known to pose as games, utilities, and email attachments. Once opened, Trojan horses act much differently than you expect. Some merely annoy, sending emails to everyone in your address book. Others do serious damage, to the point of stealing passwords and data files. Unlike viruses, Trojan horses are not self-replicating.
Active Trojan horses are an advanced type of Trojan horse. They use unprotected ports to open lines of communication with your computer, and they can ultimately give hackers control over your machine. Active Trojan horses are also called Remote Access Trojans.
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