A Trojan horse is a destructive program that comes concealed in software that not
only appears harmless, but also comes in a particularly attractive form (such as a
game or a graphics application). There may be instances when a Trojan does not have
a destructive payload. Instead, it may contain routines that can compromise the security
of your system or the entire network. These types of Trojans are often referred to
as Backdoor Trojans.
Trojans are non-replicating malware – they do not replicate by themselves and they
rely on the user to send out copies of the Trojan to others. They sometimes achieve
this by hiding themselves inside desirable software (that is, computer games or graphics
software), which novice users often forward to other users.