What is BitTorrent? Quoted from Wikipedia:
BitTorrent is a peer-to-peer program developed by Bram Cohen and BitTorrent, Inc. used for uploading and downloading files via the BitTorrent protocol. BitTorrent was the first client written for the protocol.
The concept of BitTorrent protocol is pretty simple. It uses peer-to-peer communication where a node connect directly to another node to send or receive file directly. A node can be served as a client and server at a same time. A node discover another nodes via the information from a torrent file that is normally hosted on a website. The node itself then updates itself to the torrent file too so that it can be discovered by other nodes. A torrent file contains information of the data being shared and the nodes that are sharing the data.
A node can download data from multiple nodes at different location simultaneously. Therefore, the download speed of a piece of information can be improved dramatically using BitTorrent, compared to transitional way of download via single stream. Other than that, BitTorrent also increase the availability of the information because the information downloaded by one node will be made available to others.