Trojans AKA RATs (remote administraion tools) are pretty cool tools which you can use to gain access to a remote PC. You can do lots of things to the remote PC you are controlling you can open/close cd drive, remote shell(Command line), Screen/cam campture, Passwords, Key logger, download/upload files, execute+delete files Etc.
To access a remote PC Via RATs you must execute a Server (trojan) on the remote PC Either by sending via Email or MSN etc
To access a Remote PC you must first build your server which when executed on the remote PC will connect to your PC this is called Reverse connection where you can gain control of PCs behind Routers and Networks.
When building your server you will be asked to enter your DNS/Static IP so the remote PC can connect to your PC through a Port of your choice
Using these tools does NOT mean you are a hacker because it requires no skill and is easily detected by Anti-viruses (even though you can buy a Undetectable trojans)
Unfortunately RATs have fallen into the hands of immature people who send trojans to their friends and format their drives for no reason at all.
Programmers Often spend alot of time Building these Tools and most dont get enough respect for it.
So be careful what you download it could be a trojan and a immature noob
could steal your data and your passwords