This is the screen you will see when you edit a pre-existing customer group. In the Payment Modules Box you can define which payment methods are accepted for this group. This is helpful if you want to restrict payment methods to a pre-installed module such as 30 day terms.
The second box, Shipping Modules, allows you to define which shipping modules are valid for the customer group.
The third box, Order Total Modules, allows you to define which order total modules are valid for the customer group.
In the fourth box, Exempt Group from Specific Tax Rates, allows you to define which tax rates are valid for the customer group. In this way you can have for example different sales tax rates for customer groups by defining two (say 20% and 10%) and either make the 20% or 10% exempt for a customer group.