Difference between DC & ADC

There is no difference between in DC and ADC both contains write copy of AD. Both can also handles FSMO roles (If transfers from DC to ADC). Functionality wise there is no difference. ADC just require for load balancing & redundancy. If two physical sites are segregated with WAN link come under same domain, better to keep one ADC in other site, and act as a main domain controller for that site. This will reduce the WAN traffic and also user authentication performance will increase