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.
What is HotCat and CoolCat?
HotCat is a server application used to send and receive information to/from a remote client (CoolCat) using the TCP/IP protocol. The client can this way completely control the remote computer in which the server is installed. The server application (HotCat) can serve many (thousands, theoretically) clients simultaneously. HotCat and CoolCat can be compared with other similar applications as NetBus or Back Orifice.