It's, among other things, a keylogger and password stealer. It also functions as a backdoor, hence the name.
Search for it on google and you'll find out a decent amount about it. Same goes for most any virus your AV program detects.
What does a backdoor do? It allows someone else access to your computer. Basically, anything you can do, that other person can do also.
These sorts of programs usually get installed by the user running another application, one that usually pretends to be something it's not. That's where the "trojan horse" name comes from. So, at some point, you likely downloaded something which contained this virus in it.