A trojan or a trojan horse program is a program which is usually given away for free which has a hidden purpose. The trojan program may be some type of file such as a video that user's may be interested in. The user would normally install and run this program although the installation would be so simple the user would be unaware of it. Trojan programs may or may not use a vulnerability to spread.
Trojan horse programs may send information back to the originator or may be used by the originator or attacker to gain control of a targeted system. Trojan horse programs may spread by attaching themselves to an useful program. Trojan horse programs usually attack at the application layer. Many trojan horse programs will attempt to steal user account and password information.