What are World of Warcraft private servers

A World of Warcraft server not run by Blizzard is a private server. They are essentially the same as the real thing as far as the player is concerned, except that since they are not developed by Blizzard, they tend to be less stable than Blizzard servers, lack some quests, features, and/or items, and run several release patches behind the real game (often requiring two copies of the game to be installed if you want to play both). On the plus side, they tend to have far fewer players on at once and a far lower ping time.