BitTorrent is a system that allows the sharing of almost any file over the internet, being mainly used for the distribution of videos, music and software. It is quite popular because it is an easy tool to use and at the same time very efficient.
BitTorrent itself is actually a data sharing protocol and not a centralized system. There is no server providing the data, but a standard of communication between multiple computers that allows files to be located, distributed and retrieved by all. This protocol is very much used for pirate sharing of music and movies.
