The Pirate Bay is a P2P Filesharing collective, one of the world's largest facilitators of illegal downloading, and, with its related political organization the Pirate Party, one of the most visible member of a burgeoning international anti-copyright — or pro-piracy — movement. 
From the LA Times article:
"The group's website is a database of 500,000 copied movies, TV shows, songs, games and software titles. The Pirate Bay alone claims more than 5 million active users. According to Internet traffic ranker Alexa.com, it's the 292nd most popular site in the world."
"What is the Pirate Bay? Good question, actually. Most people know the Pirate Bay’s web site, which lists torrents for countless movies, TV show episodes, albums, applications and other digital loot. Those torrent files are used as a starting point for BitTorrent-based file swapping, which means that the Hollywood flicks in question are transmitted straight from one user’s hard disk to another.
A perhaps lesser known but equally important part of the Pirate Bay’s infrastructure are its tracker servers. These help users to facilitate these BitTorrent file transfers by telling them which user has which part of the movie they want. These tracker servers currently connect around 23 million concurrent users from all over the world and facilitate the sharing of more than 1.6 million torrents. Each torrent can link to multiple files, such as all the MP3s of an entire album, which means the Pirate Bay’s users are transferring many millions of files at any given time." (http://newteevee.com/2009/02/15/the-definitive-primer-to-the-pirate-bay-trial/)
See ou entry on the Pirate Party