I think you're confused about the purpose of different types of servers. NAS means Network Attached Storage. a NAS server is a server which provides storage to client systems (which can in turn be other servers) through the network (usually TCP/IP but can be different).
A game server usually refers to the software component that mediates between the game clients to allow them to connect to each other and performs some additional administrative housekeeping choirs. A game server can also be assumed to be the physical computer on which said software in run.
Depending on the software platform used to implement a NAS server, it can also double as a game server. I don't know about the specifics for a Counterstrike Source server to help you more, but if I were in your position, I would probably head to the Valve forums.
Cheers!