From Wikipedia: A BitTorrent client is a program that manages torrent downloads and uploads using the BitTorrent protocol.
The first client, known as BitTorrent, was created by Bram Cohen, in October 2002. Cohen created the client based on what he refers to as the Golden Rule: the faster a user uploads, the faster he or she can download.
Many subsequent clients have been at least partially based on it. Not all clients were originally built for BitTorrent, having added support for the protocol later on. There have been attempts to package malware as BitTorrent clients, probably due to the availability of many legitimate clients and users willingness to try new ones.