A class group is an instance of the Data-Admin-DB-ClassGroup class.
Class groups are commonly used to cause the system to store instances of similar or related work object concrete classes together in one relational database table. This is known as a work pool(pool of workobjects had a common class referred as workgroup). Work pools are referenced in access group instances.
A class group is used as a container for storing the different case types derived from it. These case types get stored in same Database table with defined keys.
when you make a class as does not belong to class group then such instances will be stored separately.
A work pool is class name which is defined as "is a class group". one can define any number of work pools for a users access group, this suggest that the specific user can create those work pool related cases. (Note: this will only impact creation of case but it does not impact the update and modifying of cases.)
In addition to above, A work pool whose radio button is selected true will shown to user when user logins.