The technology itself is perfectly legal, but it has been debated if its implementation in connection with copyrighted material or otherwise illegal material makes the issuer of the bittorrent file, as opposed to the copyrighted material itself, liable as an accomplice or infringer. It also shares the parts you've already downloaded with others, so that everybody shares and benefits. The big file is broken down into smaller parts and it automatically fetches parts from multiple hosts at once. The BitTorrent protocol is a peer-to-peer file sharing protocol used for distributing large amounts of data.
