Kazaa Media Desktop (once stylized as "KaZaA", but now usually written "Kazaa") started as a peer-to-peer file sharing application using the FastTrack protocol licensed by Joltid Ltd. and operated as Kazaa by Sharman Networks. Kazaa is now running under license as a legal music subscription service by Atrinsic, Inc.

Kazaa Media Desktop was commonly used to exchange MP3 music files and other file types, such as videos, applications, and documents over the internet. The Kazaa Media Desktop client could be downloaded free of charge; however, it was bundled with adware and for a period there were "No spyware" warnings found on Kazaa's website. During the past few years, Sharman Networks and its business partners and associates were the target of copyright-related lawsuits, related to the copyright of content distributed via Kazaa Media Desktop on the FastTrack protocol.

Kazaa and FastTrack were originally created and developed by Estonian programmers from BlueMoon Interactive and sold to Niklas Zennström and Janus Friis (who were later to create Skype and later still Joost and Rdio). Kazaa was introduced by the Dutch company Consumer Empowerment in March 2001, near the end of the first generation of P2P networks typified by the shutdown of Napster in July 2001.