Question about Mac os X server and w2k server

Hi all . i saw server at my work place that is running windows server 2OOO. I looked at its network connections and i saw like four computer icons .Two of them were for printers and the other two had names such as dellserver and file server! What made me surprsied that in front(discription) of fileserver it was written mac os x server !! I wonder if mac os x is a hardware or sofware running on the top of windows 2000 server? I looked around i did not see any other server!! could any one explain to me how we can have such case. I know that this server provides service for both windows and macs but still i am kind of confused!I be happy if some expert explain to me how these cases are possbile .Thanks