Trojans are malicious applications that pose themselves as legitimate software in order to trick users to install them. Once on the victim's machine, it may run any number of malicious process to steal vital information or inflict damage to other software.
From Darkside Industries:
"Designed to be used with a trojan. Upload it and then run it and it'll ask for userID and password, storing them in winsys.dll in the C:\Windows directory as pain text. Download that file and open in Notepad and voila you have the password."