Wired published a very good introduction to Bittorrent. Everything that is important is packaged in this very short article:

• Background introduction
• Client softwares required: µTorrent (Windows) or Transmission (Mac)
• Popular bittorrent tracker to find files: Mininova, The Pirate Bay and isoHunt
• Basic Bittorrent jargon definition: .torrent file, seeders, Leechers, trackers
• Advanced important topics: Selective download, Bandwidth control, port forwarding
• Etiquette: Follow the Leech:seed 1:1 ratio
• Privacy:  Use PeerGuardian2 to block blacklisted IP address