There are two types of torrent trackers: Public Trackers and Private Trackers.
Public Torrent Tracker: They can be used by anyone adding them as a torrent client. You do not require an invitation or registration for this.
Private Torrent Tracker: They cannot be used by anyone. You require an invitation or registration for the same. They are safer than Public Torrent Tracker. There is a limit to your uploads and downloads.