What is a Trojan Horse?
A trojan horse is a piece of software code, usually bad, that is attached to another piece of software, apparently good. It’s named after the mythical wooden Trojan horse left outside the city of Troy full of hidden enemy soldiers.
The LdPinch-BD trojan is a good example of how this works. A spam email went out in the summer of 2005 where the attached file showed a happy animated lion singing "You are the king today! Hope you have a R-O-A-R-I-N-G time on your Birthday!" While the lion danced, a program was installed in the background that stole passwords and other sensitive data and sent them to a remote website.
How Do You Get Infected?
Websites: You can be infected by visiting a rogue website. Internet Explorer is most often targeted by makers of trojans and other pests. Even using Firefox, if Java is enabled, you’re vulnerable.
Instant message: Many get infected through files sent through various messengers.
E-mail: Attachments may contain trojans. See the paragraph entitled Precautions against Trojan horses.
.What Is It?
A Trojan horse program has a useful and desired function, or at least it has the appearance of having such. Secretly the program performs other, undesired functions. The useful, or seemingly useful, functions serve as camouflage for these undesired functions. The kind of undesired functions are not part of the definition of a Trojan Horse; they can be of any kind.
In practice, Trojan Horses in the wild often contain spying functions (such as a packet sniffer) or backdoor functions that allow a computer, unbeknownst to the owner, to be remotely controlled from the network, creating a "zombie_computer". Because Trojan horses often have these harmful functions, there often arises the misunderstanding that such functions define a Trojan Horse.
The basic difference from computer viruses is: a Trojan horse is technically a normal computer program and does not possess the means to spread itself. Originally Trojan horses were not designed to spread themselves. They relied on fooling people to allow the program to perform actions that they would otherwise not have voluntarily performed. Trojans of recent times also contain functions and strategies that enable their spreading. This moves them closer to the definition of computer viruses, and it becomes difficult to clearly distinguish such mixed programs between Trojan horses and viruses.