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 the Website:
Hack'a'Tack is a remote administration tool for Windows 95/98. As we heard from some users, the server doesn't work on NT, only the client does. Hack'a'Tack consists of two files: Hack'a'Tack.exe and Server.exe. Hack'a'Tack.exe is the Client you have to run on your own computer. Server.exe must run on the computer you want to connect to.
Once opened, the Server copies itself into the windows directory and is executed on each start of windows.