Sorta what they sound like. They’re privately owned and hosted by third party people. You have to download their own launcher to play but most are pretty much free.
Check out ascension. It’s wow but classless, you have a couple servers, one where you choose from 3 random abilities when you level up and another where you get more leeway in the abilities you chose. It’s a really interesting way to play wow.
It also good on sale for cheap relativity often. My bud picked it up for 5 bucks a couple weeks ago.