RATS. That's what they are called. Commonly known as Trojans. There is an interesting story about their origin which traces back to 12 century. The Trojan got it's name from the old mythical story about how the Greeks during the war, gave their enemy a huge wooden horse as a gift. They accepted this gift and they brought into their kingdom, and during the night, Greek soldiers crept out of the horse and attacked the city, completely overcoming it. The intention is to say that Trojans are something that are designed to look different and act differently. Now different can mean useful (very rare) or destructive (most probable).
Trojans form the major chunk of script kiddie hacking. Now what is a script kiddie? It is slang used to denote people with limited knowledge but use lamer programs to act smart and terrorize people. They have all the readymade tools at hand to make you seem vulnerable. And Trojans are their brahmastra (ultimate weapon, in Hindu mythology).
Before I elaborate more on use and functioning of a Trojan, let me enumerate a list of common Trojan ports.
Back Orifice (Default)-31337 UDP
WinCrash 1.03-5742 TCP
WinCrash 2.0-2583 TCP
Deep Throat-2140 TCP
Sockets de Troie-30303 TCP
Masters Paradise-31 TCP
Back Orifice-31336 UDP
Evil FTP-23456 TCP
Executor and HTTP-80 TCP
Note: If you don't know what TCP, UDP, etc are then please refer the earlier article.
Everyone wants to use Trojans because they let you have access to your victim's hard drive, and also perform many functions on his computer (open and close his CD-ROM drive, put message boxes on his computer etc'), which will scare off most computer users and are also a hell lot of fun to run on your friends or enemies. Modern RAT'S (remote administration Trojans) are very simple to use. They come packaged with two files - the server file and the client file (if you don't know which is which, look for a help file, a FAQ, a readme or instructions on the Trojan's homepage). Just fool someone into running the server file and get his IP and you have FULL control over his/her computer (some Trojans are limited by their functions, but more functions also mean larger server files. Some Trojans are merely meant for the attacker to use them to upload another Trojan to his target's computer and run it, hence they take very little disk space). You can also bind Trojans into other programs which appear to be legitimate.
Trojans let you play GOD. Now it feels good to have supreme power over a mere mortal's PC. But those this serve any purpose. I leave it to you to decide the answer.