Google is your friend: http://en.wikipedia.org/wiki/BitTorrent_tracker
A bittorrent tracker is some server that runs bittorrent tracker software. A bittorrent tracker is the server to which your client announces when it opens a torrent using that tracker. Your client announces to the tracker, gets the IP addresses of some peers and connects to those peers.
Some trackers also have websites associated with them where they either index or host .torrent files. Not all trackers have associated websites though. Many opentrackers don't.
Private trackers may keep track of your UL/DL statistics as well as other pertinent data.