what is the role of a member server in a domain

Hello,

I am having difficulty finding on the net more info. Curtly speaking, I am using an snap-in terminals to work on one server. In other words, many users are working on one PC simultaneously. Now I am trying to replicate this PC in case it fails, but The software which enables terminals to connect does not work on a domain controller.
My first alternative is to make a synchronization between the two PCs, but I was wondering if a member sever on which a terminal user logs could request and load user account from a domain controller.