software. The term Trojan horse is derived from a classical Greek mythology of the wooden Trojan Horse that was used to trick the kingdom of Troy and thus brought about the ruin of Troy. The difference between the Trojan horse and computer virus is that the Trojan horse does not replicate it self as the computer virus does. Therefore you cannot call it a Trojan horse computer virus or Trojan viruses. It is simply called a Trojan or computer Trojans.
Trojan horses are of two types
A useful software program is cracked and a malicious code is introduced into it by a cracker. Whenever this program is used the malicious code executes. These are usually utilities like disk scanning, clock utility, file sharing software etc.
The Trojan horse claims to be a useful program and is not. It may claim to be a game or an image file or something else. It usually tricks the user toward running it to carry out the program’s objectives.
There are many variations in Trojan horses these variations are primarily due to the function the Trojan does. Some Trojans make your systems a server and activate remote access. This variant is called Remote Access Trojan. It allows any other client access into your system. Some Trojans send out your password list files. These files are then taken by the attacker and decrypted.
Other Trojans act as key loggers that send what all you are typing to an outside system. Password if typed can be recovered from the logs of the key logger. More destructive Trojans kill the antivirus files. By deleting some key files of the antivirus, Zone Alarm, Norton antiviruses are some examples. Most Trojans download onto your PC and make it accessible to anyone who knows how to get into your PC. These Trojans are also called backdoor Trojans.
A trojan is malicious code camouflaged as something harmless, like a joke or screen saver. For example, the BackOrifice remote administration trojan (RAT) installs a stealth server that gives a remote client complete control over the infected system. Trojans like SubSeven and UseNet have been known to crash systems, delete, modify or send files to an attacker, and capture keystrokes (including passwords).