BitTorrent is a client. A ‘client’ in this case is a computer program that follows the rules of a protocol.
For example, HTTP (HyperText Transfer Protocol) is the protocol used to download web pages and other content – like this page – and your HTTP client (or browser) is the program you use to get those web pages. Some popular browsers include Microsoft Internet Explorer, Mozilla Firefox, Safari, and Opera. To an extent, they all work the same way because they follow the same set of rules.
The BitTorrent client will give you access to the world of content on the protocol in a lightweight, fast and reliable package.