BitTorrent is an advanced peer-to-peer sharing technology that runs using a client system. To share and transfer files, users must be running a BitTorrent software client on their computer. Some Web browsers, such as Opera, have built-in BitTorrent clients as part of their technology.
BitTorrent works by splitting up files into tiny bits of data that can be shared in any order. Users work together to make the file available on demand. Even if just one person has an entire file initially, eventually after sharing it with others, the speed for downloading increases.
To share and download files using BitTorrent, users must first open a torrent file that contains tracking information to get things started. BitTorrent has become quite controversial as a result of sites and services that have sprung up to track and host these tracking files (including The Pirate Bay, whose founders were recently sentenced to prison time). While the technology remains a really great way to take the burden off servers and put it on to the users, it's a hotbed for piracy of music, movies, software, and other intellectual property.