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.
PEST v4.0 RAT is an update on the earlier versions of PEST. It now has a remote batch file writer and various remote MSN messenger controls. The new client is also skinable, and comes with several preset skins, as well as the ability to create your own skins.