A Trojan Horse
Exploiting security holes in software and operating systems Windows, Unix, Solaris and Macintosh machines are all vulnerable to network attack.
Tools have been designed to give a "back door" for spying on a target computer system. Back Orifice is probably the most infamous example.
Affecting the Windows operating system, Back Orifice was a tool that could be used to monitor a remote PC covertly. Complete control was gained over the target computer, including the ability to upload and download files, read everything typed on the keyboard, and capture images from the video display.
One would be very unlucky to fall prey to something like Back Orifice, as the program needs to be installed on the target machine for it to work. It can be hidden, however, or disguised as another program... a Trojan Horse.
The moral of the story... Be very careful of what you install.