Definition: A Trojan is a self-contained, malicious program -- that is, it's a bit of software code that does something bad to your computer. It doesn't replicate (as a worm would), nor does it infect other files (as a virus would). However, Trojans are often grouped together with viruses and worms, because they can have the same kind of harmful effect.
Many of the the earlier Trojans were used to launch distributed denial-of-service (DDoS) attacks, such as those suffered by Yahoo and eBay in the latter part of 1999. Today, Trojans are most often used to gain backdoor access -- remote, surreptitious access -- to the computer.
There are several different types of Trojans, including remote-access Trojans (RAT), backdoor Trojans (backdoors), IRC Trojans (IRCbots), and keyloggers. Many of these different characteristics can be employed in a single Trojan. For example, a keylogger that also operates as a backdoor may commonly be disguised as a game hack. IRC Trojans are often combined with backdoors and RATs to create collections of infected computers known as botnets.
Also Known As: Trojan Horse