When downloading new releases of your favorite Linux distribution, you more than likely use traditional means of downloading files, such as FTP or HTTP. While there's nothing wrong with this method, you may sometimes run into problems connecting to the server.
In particular, if you're downloading the day a new release comes out, you may have a hard time connecting to busy FTP servers, or your download may be exceptionally slow due to the sheer volume of other users downloading the same files.
A relatively new technology called BitTorrent effectively solves this problem. BitTorrent uses a peer-to-peer file-sharing model that essentially boils down to everyone downloading large files, such as ISO images, in smaller pieces.
The pieces that a user downloads are then available for others to download. In effect, you're downloading from others and allowing others to download from you at the same time. After the user has downloaded all of the pieces, BitTorrent reassembles them into the original file.
The benefits are obvious. While you're using both upstream and downstream traffic to download (and upload) smaller pieces of large files, you're not grabbing these chunks from one single source. Rather, BitTorrent is essentially enacting a type of "load balancing" to obtain optimal download speeds.
You may not end up downloading a file as quickly as you would if you had sole access to a high-speed FTP site, but it's faster than sharing an overloaded FTP server with a lot of other users. And due to the peer-to-peer model, the more people involved in downloading the same files, the faster it goes for everyone.
There are a number of different clients for BitTorrent, including ones for Linux, OS X, and Windows. BitTorrent is quickly becoming a popular means for Linux vendors to distribute ISO images when new releases come out.
For more information, check out the BitTorrent Web site.