Trojan Horse or better known as a Trojan in the computer security refers to a form of suspicious software (malicious software / malware) that can damage a system or network. The purpose of the Trojan was to obtain information from the target (passwords, user habits are recorded in the system log, data, etc.), and control target (gain privileges on the target).
How it Works
Trojans differ from other types of suspicious software such as computer viruses or worms because two of the following:
1. Trojan is "stealth" (invisible and not seen) in its operations and are often shaped as if the program is a good program, while computer virus or worm to act more aggressively by damaging the system or make system to crash.
2. Trojans are controlled from another computer (the computer the attacker).
Checking Listening Ports
Detect the presence of a Trojan is an action that is rather difficult. The easiest way is to see which ports are open and are in a state of "listening", by using certain utilities like Netstat. This is because many of the Trojan runs as a system service, and work in the background (background), so the Trojans-Trojan is able to receive commands from remote attackers. When a UDP or TCP transmission is done, but the transmission from the port (which is in a state of "listening") or the address is not recognized, then it can be used as guidelines are concerned that the system has been infected by a Trojan Horse.
Sources :http://id.wikipedia.org/wiki/Trojan