A Trojan or Trojan horse is a non-self-replicating malicious program that often disguises itself as legitimate software. While some 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 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