From the Wikipedia:
BitTorrent is a peer-to-peer (P2P) file distribution protocol. The protocol was originally designed and created by programmer Bram Cohen, and is now maintained by BitTorrent, Inc.
BitTorrent is a method of distributing large amounts of data widely without the original distributor incurring the whole of the corresponding costs of hardware, hosting and bandwidth resources. Instead of the distributor alone servicing each recipient, under BitTorrent the recipients each also supply data to newer recipients, thus significantly reducing the cost and burden on any given individual source as well as providing redundancy against system problems, and reducing dependence upon the original distributor.