A BitTorrent tracker is a unique type of server that helps in the communication between peers using the BitTorrent protocol.
In p2p file sharing, a software client on is an user-end PC that requests a file, and portions of the requested file thats on a peers machine that are sent to the client, and then reassembled into a full file of the requested file. The “tracker” server keeps track of where data copies and resides on peer machines, which ones are available at time of the client request, and helps coordinate efficient transmission and reassembly of the copied file. Clients that have already started downloading a file that communicate with the tracker periodically to negotiate faster file transfer with new peers, and also provides network performance statistics; however, after the initial p2p file download is started, p2p communication can be continued without the connection to a tracker.
Since the creation of the distributed hash table ,the method for “trackerless” torrents, trackers have largely become redundant. However, they are still often included with torrents to improve the speed of peer discovery.
Learn little bit more about p2p network here
TYPES OF TRACKERS
There are mainly to types of tracker they are public tracker and private trackers
Public or open trackers can be accessed by anyone by simply by adding the tracker address to an existing torrent, or they can be used by any newly created torrent, like utorrent etc.
A private tracker is a torrent tracker that restricts it uses by requiring users to register with the site. The method for controlling registration used among many private trackers is an invitation only system, in which active and contribution of members are given the ability to grant a new user permission to register at the site, or a new user goes through a selection process to join the tracker.