"A private tracker is a BitTorrent tracker that restricts use, by requiring users to register with the site. The method for controlling registration used amongst many private trackers is an invitation system, in which active and contributing members are given the ability to grant a new user permission to register at the site"
It's pretty much a private site: examples include... jpopsuki, what.cd, waffle.fm ... there is a few others
I had to pass an interview on an IIRC to get in... Jpopsuki has an INSANE database of all things asian music (all western is banned), I duno man it has so much god damned SNSD it's not funny