depends, you can use it as a file server like i do, where you just amass large amounts of hard drives in there for storage of movies, music, appz and games as to not clog your gaming computer. You can use it as a HTTPC? i think thats how you spell it, for downloading said files and using it as a dedicated gaming server, really its just anything you can imagine using it for. I really use it as all three, and so i can still do alternate tasks while gaming.
The term server is pretty generic; it means many different things to many different people. Not sure what slots you are referring to. Possibly drive bays. If you'll post the specific case/server you are referring to then that'll help.
a generic laymans definition of a server... a computer who's primary focus is to provide a common storage area for other computers on a network to access files/apps/services.
A specific machine can be a multi role server... DHCP/SMTP/HTTP/BF2/WMA/ect....