One definition for a trojan horse that came from the White book is
This definition really is too broad and too many pieces of software will fit this definition that informed people will not consider to be trojan horses. But it really is difficult to come up with a succinct and clear definition so I will try a list of characteristics.
One challenge for a creator of a trojan horse is to entice a user to install (download) the program on their system. Trojan horses can often be hidden inside something fun such as a computer game. They can also be disguised as a special utility.
Another way for trojan horses to be propagated is through attachments to emails. This works very similar to how virus attachments get distributed across the Internet and other networks.
Hackers have tools that help them determine what systems are running remote control trojan horses. These can include communications over chat networks, emails or web pages to alert attackers when a new system has been infected and is available.
The origin of the term trojan horse comes from the Iliad by Homer where the invading Greeks pretended to be giving a prize to the Trojans within their city for having held them off. The Greeks seemed to the Trojans to have left after leaving a large wooden horse at their front gate. Actually the trojan horse had a hidden opening and space inside sit o that Greeks could hide-away in the horse. After the Trojans took the horse inside their walls and most went to sleep, the Greeks hidden inside emerged from their hiding places and let in the Greeks who has not really left.
Some Examples. Some examples of some trojan horses follow.
Protection. The single best approach to protecting your system from trojan horses is to never download an/or install that has uncertain sources, security and integrity. A good up-to-date virus scanning program should help considerably with detecting known trojan horses that have gotten installed on a system. Such virus scanning software should also help to detect and prevent installation of trojan horses.
More will be added.