From The TV IV
BitTorrent is the name of a file sharing application (and the protocol it runs on) that allows large files to be distributed quickly and easily. It has become a very popular way for users to share and download their favorite television shows; it is not uncommon for a torrent file to appear on the web shortly after an episode is broadcast.
BitTorrent works by breaking up a large file into smaller pieces. These pieces are then distributed by the "seed" (the original user) to any users that request the file. The pieces are not received in sequential order, but rather in the order that ensures the best connection. Once a piece is received by the client running on a user's computer, it may be distributed to other users who also requested the file. This eases the server load on the original uploader by farming out the work to everyone else connected to the torrent. Because the application may start uploading the file while it is still downloading, it also prevents the problem of "leeching" (always downloading and never uploading) as present in many other file sharing applications. Once every piece is received by a computer running the application, the file is reassembled for use.
The use of BitTorrent software is legal in itself, but most uses are related to sharing copyrighted material. The sharing of copyrighted material is illegal in most countries. However, a few safe havens exist which is where most trackers (a central server which links users together) are set up. One prominent example of this is in Sweden where file sharing is a large political issue, and a pro-piracy political party exists.
Partly because of the association with illegal distribution, most networks which have tried to set up legal downloading of programs have elected not to use BitTorrent to do so. It is also frowned upon since these networks are providing a pay service, and it is thought they should not be using the bandwidth of their customers. Instead they use a client/server downloading system, such as that found on iTunes.