BitTorrent is a peer-to-peer file distribution protocoll. It works by breaking a file up into pieces and then distributing them. that way a client doesn't have to download all the pieces from the same mahine.
The reference implementation is the official BitTorrent Client.
Legal Status Edit
Because there is no central server BitTorrent has avoided the legal problems that faced napster. It also helps that there is legal content available such as linux distributions