Setting up Ubuntu as a server - Help!

I installed Ubuntu desktop on my PC and want to run it as a server. I'm not finding anything that's clear to me in my searches.

Now, don't crucify me for saying this, but am I wrong in assuming that a Linux server can operate similarly to active domain, where it's setup with a domain name and users can log on with the domain name to access shares, e-mail, and centralized data? Of course I know it's not Windows server, but that's what I'm used to. I'd like to find a resource that could walk me through setting Ubuntu Desktop up as a server.

And I've heard that it can operate as a server regardless of whether or not it's desktop or server addition, you just need to install additional packages.