The file is split into hundreds of pieces. A .torrent file contains details of all the pieces.
When downloading, you can download a piece from anyone who has it. They key point about bittorrent is you can download pieces from those who haven't yet got them all - you can still download what they do have. In turn, others will be downloading from you. This makes it go faster, though the benefits are only fully realised when there are many downloaders.