The earlier versions of WordPress had a system of user levels to determine the user’s capabilities. The level assigned to the user is stored in this global variable. It is set in the pluggable functions set_current_user() and get_currentuserinfo() defined in [wordpress root folder]/wp-includes/pluggable-functions.php.
Instead of using this global variable, the function current_user_can should be used to determine the current user’s capabilities.
Back to the full list of global variables.