A Trojan or "Trojan Horse" is a non-self-replicating malicious program that often disguises itself as legitimate software. They typically intend to look as innocent and/or useful as possible to captivate people to use it, such as the infamous BonziBUDDY. BonziBUDDY disguised itself as a useful helper program, but annoyed the target, placed advertising, and tampered with settings. While Trojans have the same payloads as viruses, others open backdoor ports to allow crackers access to a remote computer and to cause the computer download programs or change settings without the user's consent.
In March of 2005, Symantec reported that trojans were on the rise, largely due to vulnerabilities in popular browsers, including the allegedly secure Mozilla Firefox and Opera browsers. Also, many of the popular worms drop trojans.
John Leyden. The Register, "Drive-by Trojans Exploit Browser Flaws". 2005.03.23