Public trackers
Public or open trackers can be used by anyone by adding the tracker address to an existing torrent, or they can be used by any newly created torrent. The most popular ones are OpenBitTorrent and PublicBitTorrent. The Pirate Bay used to run a tracker, but has since disabled it , although the website remains as a torrent index.
Private trackers
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. Invitations, typically sent via email or an invite code system, are normally granted to active users who have uploaded a pre-determined amount or meet specific upload-to-download ratio requirements. Trading invites for different sites is highly frowned upon in the private BitTorrent community as it allows anti-piracy groups to infiltrate private trackers more easily. Most private trackers monitor how much users upload or download, and in most situations, enforce a minimum upload-to-download ratio.

Some of the allure of private tracker versus a public one are: higher speeds, a tighter community, and safer downloads. Private trackers implement a strict set of rules, so generally files containing malware are extremely uncommon. Many private trackers keep in close contact with each other, so bad users (who trade invites or attempt to fake their ratio) can be quickly blacklisted. Almost all private trackers implement a passkey system, where each user is given a personalized announce URL so if there is unauthorized distribution, it can be pinpointed to the user responsible. Some private trackers have a higher level of security than others; many sites only allow their users to refer to their site as an abbreviation, and never as the site's full name or URL. Other trackers restrict invites to outstanding members, and many trackers, to increase security, have eliminated the invite system altogether. An example of a private tracker is Oink's Pink Palace, which was forcibly shut down in late 2007 by law enforcement officials.

The downside is that in a closed community it can be hard for all members to maintain the required ratio. Competitions may be offered, with prizes of improved ratios. Some trackers will exchange ratio credit for donations. Some trackers will also use "free leech" systems to improve the users ratio. When downloading a free leech torrent only the upload gets logged in, the download is ignored. Usually large torrents are offered as free leech. Seeding to a ratio of at least 1 is still recommended even for free leech torrents.

Trading invites for different sites [...] allows anti-piracy groups to infiltrate private trackers more easily.

I call that bullshit

thanks for information