Create Plugin, Elementtype suggestion
You likely won't need to write a plugin at all – you can easily create different user groups (e.g. "Workers" and "Clients") from the Control Panel, and grant those groups different permissions. An example of a permission is "Access the CP". The user group permissions will be automatically applied to any new users added to the group.

To create a user group, visit Settings -> Users in the CP. The edit form for a user group will have a plethora of checkboxes for every possible permission. Note that when checked, many of these checkboxes will actually reveal even more checkboxes, for even more fine grained control over permissions.