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).
Transmission - a popular and lightweight BitTorrent client, providing GTK, QT, command-line and web interfaces.
bittorrent-gui - A BitTorrent-GUI.
Get Debian ISO images with Bittorrent
A comparison of bittorrent clients