The users module allows you to manage users that can access the Control Panel for updating your web site. The users for accessing the Control Panel are completely separate from the users in your user directory; users in your user directories can not access the Control Panel.
You can control which areas of the Control Panel users can access using user roles. Each user role can access whichever modules you choose, and each user role can have any number of users. When you create a new Control Panel user you select which role the user has.