What is a Private Tracker
A Private Tracker consists of a private site and torrent tracker that provides the same functionality as a public tracker but is invite-only.
This means you need to be a member to view the contents of the site and download its torrents.
There are 2 main types of Private Trackers:
Semi-Private - a user may sign up and login to the site without an invite.
Private - a user must be invited to create an account either via other users, recruitment from other trackers or an interview.
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.
What are the advantages of a Private Tracker?
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 increasing the chances of a torrent having a healthy swarm for longer. Not only that, but many members use seedboxes, which are just servers based in datacenters, offering very high speeds and excellent peering. Enough to max out anyone's home connection.
Retention: Similarly, Private Trackers usually enforce rules that encourage long-term seeding. A few peers will still seed many torrents for obscure content that you wouldn't normally find any peers for on many public trackers.
Selection: Some content simply isn't available on any public site and will only be found on Private Trackers. Some trackers specialise 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 stringent rules on the content format, quality, and organisation.
What are the best Private Trackers?
There are two kinds of trackers, content-wise: specialised and general. General trackers will often serve the majority of your needs, bigger general trackers will usually have plenty of niche content. Many popular public trackers are general trackers, RARBG, rutracker and 1337x for example. Due to their broad scope they will still never match the niche trackers in their particular area of specialty. The following is a global consensus on what is considered the best tracker in their fields, namely: Music, Movies (mainstream and obscure), TV, HD, Games, Books, E-learning, Porn, Anime, Scene and General.
Audiobooks: MyAnonaMouse (note that the best GENERAL source of audiobooks is AudioBookBay and occasionally RuTracker which are not Private Trackers)
Piano sheets: MyAnonaMouse
E-learning: TheOccult (esoteric), TheVault (business), TheShow (entertainment), ThePlace (seduction)
Obscure/Older TV: KaraGarga, cinemaTIK, SecretCinema, MySpleen, TV-Vault
Hentai: Sukebei (Public Tracker), AnimeBytes
Porn: M-Team TP (JAV), EMPornium (General), ExoticaZ (Asian), Sinderella (Korean), Sukebei (Public Tracker, JAV)
Asian Music: JPopSuki
Visual Novels: Sukebei (Public Tracker), GazelleGames, AnimeBytes
Rhythm Games: Bemaniso, OnlyOneCab
3D Software, Graphic Design, VFX: CGpeers, GFXpeers
r/Trackers Wiki - Has some helpful guides for people new to Private Trackers, including guides on how to get in them.
Private Trackers Guide - Some info about Private Trackers by r/Piracy.
Private Tracker List - A helpful list that has updated stats of most of the Private Trackers.
InviteRoute - Shows which Private Trackers you can enter from which other ones.
r/OpenSignups - Private Trackers that are currently having open signups get posted here, occasionally you get good ones.
Here is a list of the homes of various Internal Groups in Private Trackers.