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.