Well, the first question is too broad to answer directly. Could you be more specific?
As for the second question, that will depend on the OS the server is using. However, a server is generally going to have a regular OS with stuff added on to it. So yes, you can use it like a regular computer for the most part. It should be noted that "can" and "want to" are different concepts. Many servers are actually better left in a closet and ignored apart from regular maintenance.
It may help if I observe that the opposite of a server is a client. The client wants stuff and the server makes it happen. You could think in terms of a restaurant. The client wants the chicken and the server brings the meal to the table.
Then too, there are all different kinds of servers. If you want to meet me for an online game someday, I could use my computer as the main one that the game is running on (server) and you would connect to my box (client) to interact with me. We would both be playing the game together but I would be the guy who got to decide what maps are going to be available and which cheat codes you can use. As the server admin, I could even force your copy of the game to run certain variables (like “big heads” in UT).