Again, just as with viruses and worms, there are many types of Trojan programs - the most popular being “Back Orifice”, “Netbus” and “SubSeven”. Depending on the type of trojan installed and the motives of someone who gains access to an infected machine, the results can be disastrous: trojans can allow someone to see what you are seeing (on their screen), to transfer files from your computer to theirs, to delete your files and crash your computer, to use keyloggers to track (log) what you type in order to steal passwords/bank account/credit card info, to open and close CD-ROM drives, take control of your mouse, turn the machine and monitor on/off, and much more.
Almost all trojans will attempt to open a “port” (metaphorically, an open port is like an open door to a house) to broadcast the presence of an infected machine to “port scanners” (people looking for open ports on infected machines, to break into). Some trojans are also programmed to establish a direct connection to a specific person/computer, or to commit illegal DoS (Denial of Service) attacks on specific websites. Firewalls (see futher in this document) are particularly useful to block Trojans from trying to access the Internet, and from people trying to gain access to your machine.