Group policy provides the stream line access to all the users in the network, it can be used to define the users security and networking policies. By group policy you can keep control over certain function like not allowing the users to shut down the system or using the control panel or running the command. Group policy is applicable on active directory container like OU, site and Domain.
There is one difference between domain controller and additional domain controller, on DC all five operational roles are available while in ADC only three operational role is available.
When one or more computers work together as a single system by sharing their resources is known as clustering. The benefits of clustering are that it reduces the load on one single system by sharing the load also it is used for redundancy of the services.