A Trojan horse is malicious software that gets installed into your system software in order to create remote backdoor access into your computer. Trojan horses are typically assisted by the user in getting themselves installed, often via files downloaded from the Internet. The Trojan horse is not technically an attack, but opens up an access point for other malicious software (like worms) or for malicious hackers seeking access to your system.
These are programs that are designed and written to look like normal, useful programs, but contain hidden code that can perform a wide variety of compromises up to and including granting a remote user complete control of the compromised computer.7
Trojans are not limited to desktop computers, but can also infect mobile phones and other mobile computing devices.
How do I protect myself against Trojan horses?
The best protection against Trojan horses is being careful about the software you install on your computer. Since Trojan horses need your help, your job is to not help them. That means you only install software on your system when you have absolute confidence in its integrity.