Basically your are being held responsible for seeding it either 1:1 or for some time if you cant reach that goal.
Longevity is rewarded on some trackers with more bonus points you can trade for stuff like Upload credits. The longer you seed, the more you can get per seed.
Usually privates can get exclusive stuff which trickles to public trackers but they also pull from public (like anime from nyaa) and reseed them here. Some private tracker users are unable to download from public because either their ISP watches that more closely or their seedbox provider does not permit it/banned connections
The general quality is >=720p and usally a higher bitrate than public ones.
Sime trackers are general (like 1337x and TL), specific for movie and tv (like RARBG and Blu) and some others for topics like ebooks and audiobooks, music/-videos, learning materials.
Some may even only allow content that was produced >10 years ago (cathode) or something like cartoons (old toons) or culture specific (like jptv and nyaa) only.