Bittorrent is a P2P protocol designed for sharing files over the Internet and was created by Bram Cohen in April 2001.
Is a system for downloading files between computers for parts, so do not need to be available whenever a server to download the entire file completely. This protocol works by creating a file containing a unique identifier for each file, these are published in a list called "tracker" that contains the list of computers that are downloading the file at that time.
There are two types defined for these customers:
seeders: people who have downloaded the content and shares the same
leechers: those who only download it, either because they have no parts or unwilling to share when finished.
The trackers are differentiated between "public" and "private." The public are the ones who can access them without any registration, while in private no.
A good example of each is demonoid.com (private) and thepiratebay.org (public). Many of these have their files accessible, which can be downloaded without any problems should they meet the requirements requested.
There are other pages that do not have trackers, however, contain an incredible amount of files in torrents search engines such as Mininova.org or IsoHunt.com.