Some of my mobile games ask you to create a username so that your high scores can be logged on a global leaderboard not located on your device. This information — your created username and your game score — is transmitted from your device to a remote web server. Other users of the game can view this information on the high scores page of the game.
No other data from the device is transmitted and stored – just the username you create (e.g. “gamer2112”) and your score.
So in other words, be smart and don’t enter your name, address, and social security number as your username.