How BitTorrent it works:
It [BitTorrent] divides shared data (a single file or a directory) into pieces, typically of 256 KiB. A SHA-1 checksum is computed for each piece, and used to check the piece has been correctly downloaded. The checksums are stored in a .torrent file, along with filenames. The .torrent file also nominates a tracker, a Web resource that introduces peers to each other. Peers contact each other, learn what pieces they have available, request the rarest (least commonly seen) pieces first, and send requested pieces.