A name checker is a computer program that reveals the availability of usernames for popular online games such as Minecraft. Some also allow users to trace the naming history of a particular player.
While players can change their usernames with few restrictions, the game must be able to recognize the player regardless of what they are currently calling themselves. This is accomplished through a Universally Unique Identifier (UUID), a never-changing identifier that looks like gibberish if written out. This allows the game to continue to recognize the player and also allows name checkers to trace the username history of any particular account. Some name checkers reveal the UUID of players as well as past usernames.