Back Orifice 2000 is a back door program that lets you remotely control a computer running Windows through TCP/IP. Although it could be used for legitimate purposes, it is most often used for cracking. When connected to a BO2K server, a BO2K client can download files, upload files, delete files, edit the system registry, make dialog boxes pop up, reboot the computer, and many other things. Many useful plugins have been written for BO2K, including several good encryption plugins, a few that send an email containing the IP address of the BO2K server when the computer with the server on it connects to the internet, and one that will let you see a video stream of the server's screen, among many others. There is also a very useful program that combines a BO2K server program with another Windows executable, allowing people to disguise the server as a legitimate program that does something (instead of the normal BO2K server installation program, which appears to do nothing but delete itself). BO2K can be annoying however, when the l33t script kiddies use it as their reason for being "hackers". BTW, the default port for BO2K is 31337.