The Trojan horse, also known as trojan, describes a class of computer threats that appears to perform a desirable function but in fact performs undisclosed malicious functions that allow unauthorized access to the host machine, giving them the ability to save their files on the user's computer or even watch the user's screen and control the computer.

Trojan Horses (not technically a virus) can be easily and unwittingly downloaded. For example, if a computer game is designed such that, when executed by the user, it opens a back door that allows a hacker to control the computer of the user, then the computer game is said to be a Trojan horse. However, if the computer game is legitimate, but was infected by a virus, then it is not a Trojan horse, regardless of what the virus may do when the game is executed.

Types of Trojan horse payloads:

Trojan horse payloads are almost always designed to cause harm, but can also be harmless. They are classified based on how they breach and damage systems. The six main types of Trojan horse payloads are:
Remote Access
Data Destruction
Downloader/dropper
Server Trojan(Proxy, FTP , IRC, Email, HTTP/HTTPS, etc.)
Disable security software
Denial-of-service attack (DoS)

Methods of deletion

Since Trojan horses have a variety of forms, there is no single method to delete them, there are many different ways instead. The simplest responses involve clearing the temporary internet files file and deleting it manually. Normally, antivirus software is able to detect and remove the trojan automatically. If the antivirus cannot find it, booting the computer from alternate media, such as a live CD, may allow an antivirus program to find a trojan and delete it. Updated anti-spyware programs are also efficient against this threat. Most trojans also hide in registries and processes.