In the Webster dictionary a torrent is defined as a flood or rush of words, mail, and in this case music, movies, etc. In order to use a torrent file you must however, have a Bit Torrent client to download the torrent extension files. Sounds a little confusing at the moment, I know, it's not though. You're looking at the word's more than the big picture.
Bit Torrent is just a basic set of rules and description of how to do things to help you and allow you to quickly download files as well as distribute the file being downloaded at the same time. Most of the time when using this application, large files are downloaded or distributed.