Historically, computer trojan horses have been defined as a class of infiltrations which attempt to present themselves as useful programs, thus tricking users into letting them run. But it is important to note that this was true for trojan horses in the past–today, there is no longer a need for them to disguise themselves. Their sole purpose is to infiltrate as easily as possible and accomplish their malicious goals. “Trojan horse” has become a very general term describing any infiltration not falling under any specific class of infiltration.
Since this is a very broad category, it is often divided into many subcategories. The most widely known are:

• downloader – a malicious program with the ability to download other infiltrations from the Internet.

• dropper - a type of trojan horse designed to drop other types of malware onto compromised computers.

• backdoor - an application which communicates with remote attackers, allowing them to gain access to a system and to take control of it.

• keylogger – (keystroke logger) – a program which records each keystroke that a user types and sends the information to remote attackers.

• dialer - dialers are programs designed to connect to premium-rate numbers. 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.

Trojan horses usually take the form of executable files with the extension .exe. If a file on your computer is detected as a trojan horse, it is advisable to delete it, since it most likely contains malicious code.

Examples of well-known trojans are: NetBus, Trojandownloader.Small.ZL, Slapper