BitTorrent is a file distribution system used for transferring files across a network of people. As you download a file, BitTorrent places what you download on upload for other users; when multiple people are downloading the same file at the same time they upload pieces of the file to each other. BitTorrent pieces together the file you are downloading, to where the first part of a file you get may be the last part someone else gets. As you continue to retrieve the file, BitTorrent also uploads data to other users. For example, a person with 98 percent of the file done is directed to the people with the 2 percent of the file they still need.

The files downloaded via BitTorrent are called torrents.

BitTorrent
The Official BitTorrent Web site.