A contribution by Sepp Hasslberger:
BitTorrent is a method of file transfer that allows the division of large files into manageable pieces, which can be individually downloaded from “seeders”, i.e. those users that publish (or ‘seed’) such files for others to download. BitTorrent trackers are sites that track who is currently on line, offering which pieces of a large file for download. Trackers are needed to efficiently distribute the workload of serving up large files, and to match what is being offered at any given moment with the bit torrent users requesting the content.
Most trackers are not ‘pure’ trackers, in that they also offer a searchable index of the files being shared. It is those indexing trackers, like the PirateBay, Mininova and isoHunt, that come at times under legal attack because some of the files that are being shared through them are copyrighted material shared without permission of the copyright holder.
Open BitTorrent trackers (such as PublicBitTorrent, OpenBitTorrent, and HiddenTracker offer torrent tracking to anyone without strings attached and without being part of a vulnerable torrent database. They do not keep files or even know what is being shared.
A bit torrent file has to be indexed by at least one tracker, if downloading is to be efficient. Trackers make it possible to use the resources of the original file sharer as well as those users who, while themselves downloading, pass on pieces of the file to other users.