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 https://packages.debian.org). The list includes:
Transmission - a popular and lightweight BitTorrent client, providing GTK, QT, command-line and web interfaces.
bittorrent-gui - A BitTorrent-GUI.
aria2 - High speed download utility (command-line)
https://www.debian.org/CD/torrent-cd/ - Get Debian ISO images with Bittorrent
http://en.wikipedia.org/wiki/Comparison_of_BitTorrent_clients - A comparison of bittorrent clients - Wikipedia