BitTorrent is a relatively new method of file-sharing. Like the Gnutella network it is decentralized. However the differences end there.
When downloading a file using bittorrent, you initially download a very small file (a .Torrent file) which will initiate the connection of the file you wish to download. It then creates a temporary peer-to-peer network with those people also downloading the same file so that when you are downloading pieces of a file from someone you are also uploading the pieces you have to those who don’t have it. There are several advantages to this one is the burden it takes off the server who originally hosted the file. The second and most important is that the more people downloading a file the faster it gets. Red Hat Linux sponsored the development of Bittorent as that is how you download their distro from them in the future.
There are several downsides. The small .Torrent files are hosted on websites so it takes a bit of googling to find what you want. A search for Bittorrent links is a good start.
Bittorrent clients are available for Mac, Linux and Windows.

The Bittorrent file distribution model