Torrent files are BitTorrent files. Go to versiontracker.com to DL the software.

BitTorrent turns a (usually large) file into an indexed file that allows it to be shared by large numbers of users. The users start with a "seed" file (with extension .torrent) that starts off the sharing with anybody else linked to the file. They can break off and continue sharing at any time.

All users share the file downloaded from anyone else who has more of the file and uploaded to anyone who has less. By multiple sharing the transfer can be reasonable quick and smooth.

Obviously it depends on larger numbers of people sharing and staying connected well past the point of completion so that others may also receive the file.