Username of the current WordPress user is stored in this global variable. It is set using the cookie in the pluggable functions
set_current_user($id, $name='') and
get_currentuserinfo() defined in [wordpress root folder]/wp-includes/pluggable-functions.php.
It is used to get userdata using the username., e.g.,
$userdata = get_userdatabylogin($user_login);. The plugin and theme authors need not use this, as the user data is already available in the current_user and other global variables..
Back to full list of global variables.