A Trojan Horse is a program intended to perform some covert and usually malicious act which the victim did not expect or want. It differs from a destructive virus in that it doesn't reproduce, (though this distinction is by no means universally accepted).

Trojan Horses are used to infect a pc then be able to control it remotely. A good example would be infecting a pc at the library and being able to go home and open its cd rom drives, see what web sites are opened, see what keys people are typing and being able to shut it down. Basically control it 100% Trojans are programmed to run hidden so the pc owner wont know its there.

The other descriptions describe what it is, this is how it started. It comes from the legendary battle of Troy, where the Ancient Greeks were attempting to get into the heavily protected city of Troy. According to the legend, they built a large wooden horse, put soldiers in it and left it outside Troy. The people of Troy thought it to be a gift, so they took the horse inside. The soldiers then came out at night, opening the gates so that the Greeks could enter and destroy Troy. According to legend. Trojan viruses are similar, allowing the hacker 'in'.

A Trojan Horse is a program intended to perform some covert and usually malicious act which the victim did not expect or want. It differs from a destructive virus in that it doesn't reproduce, (though this distinction is by no means universally accepted).

In the world of computers, a Trojan horse, (also known as a "trojan"), is a piece of malware which appears to perform a certain action but in fact performs another. Contrary to popular belief, their payload, may or may not actually be malicious. Trojan horses are notorious today for their use in the installation of backdoor programs. Simply put, a Trojan horse is not a computer virus. Unlike a virus, it does not propagate by self-replication but relies heavily on the exploitation and deception of an end-user. It is a categorical attribute which can encompass many different forms of codes. Therefore, a computer worm or virus may be delivered by means of a Trojan horse. The term is derived from the classical story of the Trojan Horse, which carried a hidden payload. Types of Trojan horse payloads Trojan horse payloads are almost always designed to do various harmful things, but can also be harmless. They are broken down in classification based on how they breach and damage systems. The six main types of Trojan horse payloads are: - Remote Access Software - Data Destruction - Downloader Programs that download programs without the users knowledge. - Security software disabler - Denial-of-service attack (DoS) - Server Trojan (Proxy, FTP , IRC, Email, HTTP/HTTPS, etc.) Some examples of damage are: - Opening and closing CD-ROM tray - Playing sounds, videos or displaying images. - Making screenshots - Randomly shutting off the computer - Harvesting e-mail addresses and using them for spam - Restarting the computer whenever the infected program is started - Deactivating or interfering with anti-virus and firewall programs - Deactivating or interfering with other competing forms of malware - Copying fake links, which lead to false websites, chats, or other account based websites, showing any local account name on the computer falsely engaging in untrue context - Showing fake downloads of software movies, games, porn videos and porn websites, that you did not download nor go on. - Spying on the user of a computer and covertly reporting data like browsing habits to other people - Calling using the modem to expensive numbers, thus causing massive phone bills. - Logging keystrokes to steal information such as passwords and credit card numbers - Phishing for bank or other account details, which can be used for criminal activities - Installing a backdoor on a computer system - Erasing or overwriting data on a computer - Encrypting files in a cryptoviral extortion attack - Corrupting files in a subtle way - Upload and download files - Allowing remote access to the victim's computer. This is called a RAT (remote access trojan) - Spreading other malware, such as viruses: this type of Trojan horse is called a 'dropper' or 'vector' - Setting up networks of zombie computers in order to launch DDoS attacks or send spam. - Installing a virus Source: http://en.wikipedia.org/wiki/Trojan_horse_%28computing%29