Bit Torrent is a file sharing protocol. Instead of downloading files from one server, we download (leech) from each others computers, and, at the same time, give(seed) for them.
Its like one big file is split into 10000 parts and then we download in parts from other's PCs. The torrent client will then join all those parts to make one complete file.http://en.wikipedia.org/wiki/Peer-to-peer
Most Linux distros will come with a torrent client -- Transmission, Ktorrent, azearus etc...
Torrents are very useful to get free stuff. Just try browsing http://www.thepiratebay.org