BitTorrent has been popularized by sites such as ThePirateBay. These sites are considered public BitTorrent sites due to them providing metadata to download files using the BitTorrent protocol to any user who visits the site and people to register with no restrictions to share metadata with other users. This opens them up to malicious activity by law enforcement agencies, the **AA companies and others including malware authors and people who may disagree with the site or its users. The content on these sites is not held to standards that could prevent users from downloading malicious software and depends on their (the users’) ability to distinguish these files.
Private trackers are BitTorrent trackers that often require the user to meet a set of prerequisites before being allowed access to download or upload content. This could range from having a good (positive, or within a certain amount) ratio on other, public sites, that may track such statistics, or have them be invited by other users. Some trackers are more exclusive than others with things such as aptitude or knowledge tests before being invited by an Administrator or user with the authority to make decisions on requests.
The content on private trackers is generally held to much higher standard and maybe pertain to a specific interest such as music, movies, games or be a more general purpose site that could offer of the above to its users. These types of sites generally have less concern with law enforcement or other malicious actors monitoring and interfering with their content and users and as such are seen as a more preferable way of downloading torrents.