Bit torrent is file sharing or p2p protocol. It enables sharing of files easily thru the use of a torrent file. The torrent file contains meta data about the tracker and the file. The best thing abt BT is that users have to upload files simultaneously as they download

...this ensures that everyone gets the file. In other p2p like kazaa it's not complusary to share but in BT its is mandatory and certain site also enforce a share ratio. A share ratio is amt uploaded:amt downoaded, this shud ideally be greater than 1, some sites wil ban you if u keep leeching and your share ratio is below 1.
the files are split when downloading in BT and random parts are downloaded thats y you cant view partially downloaded files in Mediaplayer.
If you need a more detailed FAQ go here BT FAQ