What is a private tracker?

A private tracker is a torrent website that provides the same functionality as a public tracker, but is generally invite-only. This means you need to be a member to view the contents of the site and download its torrents, but you cannot register to be a member without having been invited by an already existing member (who in turn must be authorized to give out invitations). Within a tracker, there is usually an extensive set of rules covering how much one can download, what kind of content one can upload, what precautions one must take when logging into the site, etc. Such rules and content vary from tracker to tracker, and go from rather liberal with little enforcement to ultra-paranoid and autistic.

Advantages of private trackers include:

Speed: If you're familiar with torrents you probably know that the bigger the swarm, the faster you download. Private trackers encourage their members to seed torrents for as long as possible, thus guaranteeing bigger swarms for every torrent. Not only that, but many members use remote seedboxes with huge upload speeds, thus dramatically increasing the speed for many torrents, often making them available almost instantly.

Retention: Similarly, private trackers usually enforce rules that encourage long-term seeding. Many torrents for obscure content that you wouldn't normally find any peers for on ThePirateBay or KAT will be still seeded by a few peers on private trackers.

Selection: Some content simply isn't available on any public site and will only be found on private trackers. Sometimes you can't even legally buy it at all, ironically. Some trackers even specialize in obscure or rare content, ensuring that it doesn't get lost from the Internet.

Quality control: A major asset of private trackers, albeit one that can vary a lot across trackers. Good private trackers have very strict rules on content format, quality and organization. Music trackers will ensure you don't get horrible 92kbps transcodes, movie trackers will ensure you only get good encodes, ebook trackers will ensure you get retail quality, etc. Members and staff review and approve each torrent. Trumping rules and the removal of duplicates ensure you only get one, community-approved source for the specific content and format. This, coupled with a decent site layout, makes private trackers much more orderly than public ones.

Security: There are two reasons private trackers are more secure, albeit they may not apply in the future. The first one is that most of them are obscure enough that no one really knows or cares about them (security by obscurity). The second one is that copyright trolls would rather focus on huge public sites that are easy to fish for peers rather than small communities that are hard to join. From a monetary point of view, it's more worthwhile to stop 10,000 casuals from downloading two torrents than to stop two neckbeards from downloading 10,000 torrents.