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.
As reported from Symantec:
Backdoor.WinShell.50 is a server program that allows unauthorized access to an infected computer. The Backdoor will listen on port 8719.
This piece of malware, along with Trojan.Stealther.B, has recently been found on systems that the Microsoft DCOM RPC vulnerability has exploited.