Torrent Files and Client

A torrent file stores metadata used for BitTorrent. It is defined in the BitTorrent specification.
In a conventional Internet download, a file or group of files is transmitted from one computer (usually a server) to another computer (a client). Data is held in a central location and is retrieved per the request of a single user.
A torrent, however, distributes data differently on many levels. Firstly, a .torrent file is not the actual file being retrieved. The .torrent file is merely data which has information about the file that the user is seeking. The .torrent file is, in a sense, a sort of index of the file being retrieved. The torrent divides the target file into a series of equally sized pieces that are each assigned an identifying checksum.
The transfer of the torrent file is done between many different peers. Rather than the data being transferred from one machine to the next, the various pieces of the file are held on many different computers. The torrent client communicate with other peers in order to check which pieces they have and exchanges them for the ones they need. In this way, transferring files via a torrent is sort of like a lightning fast, digitally automated game of "Go Fish." Peers swap pieces of files with as many different computers as possible and eventually combine the various parts into the whole, requested file.
A torrent file contains the URLs of multiple trackers and integrity metadata about all the pieces. It can also contain additional metadata defined in extensions to the BitTorrent specification. These are known as "BitTorrent Enhancement Proposals". Examples of such proposals include metadata for stating who created the torrent, and when.
BitTorrent is a peer-to-peer file sharing protocol used for distributing large amounts of data. BitTorrent is one of the most common protocols for transferring large files, and it has been estimated that it accounted for roughly 27% to 55% of all Internet traffic (depending on geographical location) as of February 2009.
Programmer Bram Cohen designed the protocol in April 2001 and released a first implementation on July 2, 2001. It is now maintained by Cohen's company BitTorrent, Inc. There are numerous BitTorrent clients available for a variety of computing platforms.