BitTorrent is a peer-to-peer file sharing protocol used for distributing large amounts of data.
Debian provides many BitTorrent clients. Use apt-cache search bittorrent to get the list (or search on http://packages.debian.org)
For example:
bittorrent-gui - A BitTorrent-GUI
rtorrent - A console based BitTorrent, with curse interface.
External Links
Get Debian ISO images with Bittorrent
http://www.debian.org/CD/torrent-cd/A comparison of bittorrent clients
http://en.wikipedia.org/wiki/Comparison_of_bittorrent_clients