Historically, computer Trojans (Trojan horses) have been defined as a class of threats which attempt to present themselves as useful programs and thus trick users into running them.
Since Trojans are a very broad category, it is often divided into several subcategories:
•Downloader – Malicious programs with the ability to download other threats from the Internet.
•Dropper – Malicious programs with the ability to drop other types of malware onto compromised computers.
•Backdoor – Malicious programs which communicate with remote attackers, allowing them to gain access to the computer and take control over it.
•Keylogger – (keystroke logger) – A program which records each keystroke that a user types and sends the information to remote attackers.
•Dialer – Malicious programs designed to connect via premium-rate numbers instead of the user's Internet service provider. It is almost impossible for a user to notice that a new connection was created. Dialers can only cause damage to users with dial-up modems, which are no longer regularly used.
If a file on your computer is detected as a Trojan, it is advisable to delete it, since it most likely contains nothing but malicious code.