A trojan is a back door program, enabling a certain port / ports to open when attempted to.

Chances are this is just a script kiddie, using something simple, like Back Orifice, Sub 7, or netbus... All of which are programs for network administration and allow for connection via a network ( the internet is one enourmous network).

Some trojans are capable of sending out information, such as IP address, open port numbers and things to that effect.

Chances are, if you have this on your system, the person who ACTUALLY posted the link, will never even access your computer, and someone else will, unless of course, the person with the trojan passworded it.

Roughly in one hour, your computer will be scanned at least 10 times by random locations across the globe, showing really, that kids now a days are a bunch of loosers.

My advice to you, is if you ARE infected, or think you ARE, use an antivirus program, or use a free one ( search google for trend housecall ) it's free and pretty good.

Oh and for clarification, I know not EVERY kid does it, it's just mainly done by teenagers seeing as someone who actually knew what they were doing wouldn't need to use a trojan, for example, I'm 21 and could quite easily get someone's IP address and trace them, for example.
But I wouldn't do that