What is Delegated Administration and what is the difference between Administrator and Delegated Administrator?

Delegated Administrators are non admin users who have been assigned some admin privileges and can perform certain admin functions.  Delegated administrators can: Create and edit users Unlock users. Assign users to specified profiles. Assign or remove permission sets for users in their delegated groups. Create public groups and manage membership in specified public groups. Log in as a user who has granted login access to the administrator. Manage custom objects and customize nearly every aspect of a custom object.  However, Delegated administrators cannot: Assign profiles or permission sets with the “Modify All Data” permission Don’t see the None Specified option when selecting a role for new users Need access to custom objects to access the merge fields on those objects from formulas Can’t modify permission sets Must be assigned the "Manage Roles" permission to change the role of portal account owners

The difference is that delegated administrators have only some of system administrators privileges.