BitTorrent is...

...the name of a peer-to-peer application and file format protocol.

But what makes it different from other file-sharing applications?

For one, BitTorrent doesn't use the client-server model where files are hosted on a central server and distributed to those clients that request them. Secondly, unlike other popular P2P applications such as KaZaa, Gnutella, DC++, etc., BitTorrent is not an application where all file searching is done through a self-contained interface. Instead, searches are done strictly over the web.