It’s been mentioned elsewhere on the site and there is also another thread for it (which I don’t like), but I’m starting another. Why? Because someone asked me to. This should be everything you need to know.
Here’s the official site for explanations about how it works.
Here’s the client I use (and think is the best)
This software is great for serving big files because instead of one computer serving many, on serves to a few and those few serve to each other and everybody shares with everybody. So basically the more people downloading (and uploading) the faster your speeds are, as opposed to more people downloading equaling slower speeds.
Here’s a place you can test it to get your feet wet.
There are many of these torrent sites, just do web searches or whatever to find them.
And for any naysayers to high download speeds from bittorent, I often get 180KB/sec+.