Save a new or updated user.
$account: The user object to modify or add. If you want to modify an existing user account, you need to ensure that (a) $account is an object, and (b) you have set $account->uid to the numeric user ID of the user account you wish to modify. If you want to create a new user account, you can set $account->is_new to TRUE or omit the $account->uid field.
modules/ user/ user.module, line 335
- Enables the user registration and login system.