Definition:
A Trojan horse is a malicious program that attempts to trick you into installing it by pretending to be something else. If it's successful, the Trojan can then carry out whatever task it was designed to do, such as provide unauthorized access to, and control of, your computer.
The application the Trojan installs is known as the payload. The payload may do something as innocuous as display a simple greeting onscreen, but it's more likely to do something malicious, such as delete all of your files. The most common payload is a backdoor, an application that provides its creator with unauthorized access to, and control of, your computer.
Examples:
A Trojan horse is usually designed to cause damage, such as delete all of your files or provide unauthorized access to your computer.
