BitTorrent is a peer-to-peer protocol for sharing large files across the Internet or IP networks developed in 2001. It is used by many open source projects as it reduces bandwidth costs, as users share the file using their own bandwidth. It can therefore be efficient and speedy.
It also has the advantage of allowing downloads to happen in partial bursts, so large files do not have to be downloaded in one session.
A BitTorrent Tracker is a computer that coordinates the file distribution among the clients, but may not necessarily have the content of the file itself.