BitTorrent Trackers are Hyper Text Transfer Protocol (HTTP) or User Datagram Protocol (UDP) servers that keep a list of peers associated with an info_hash and return that peer_list when queried. Thus, at their core, trackers have no knowledge of the files clients are exchanging. Some trackers are configured to only keep peer_lists for certain info_hashes (registered torrents) or to only return the peer_list to certain query requests (registered peers). In common parlance, some websites operate as front-ends for tracker software so are referred to as "trackers." This is technically incorrect, but this diction shall be used going forward.
Prerequsite: Introduction to BitTorrent Protocol, Beginner's Guide, WikiBooks, Wikipedia
