BitTorrent is a peer-to-peer (P2P) file distribution tool written by Bram Cohen.

Unlike traditional file sharing systems, its primary intention is to provide an efficient way to distribute the same computer file to a large group of people by having everybody that downloads a file also upload it to others. First, a small file with a .torrent extension is distributed via conventional means. This file is static, so it is often placed on regular websites or even distributed by e-mail. The .torrent file contains hash function information for blocks of the file, so the size of it depends on the size of the file or group of files that it refers to. It also contains, hardcoded, the address of a so-called "tracker server" which is used to locate sources that have the file or parts of it.

You can see more info in this external link.