I've never really seen Mac OS X in different "flavors" like windows, does it have them? Or does Apple just give you one straight OS, as opposed to microsoft splitting up the OS into different tiers:
There are two, as nagul pointed out. What differentiates them is that OS X Server provides a graphical interface to system services that you don't get in OS X. OS X Server also has things like an iCal server and Jabber server built in, wiki software, and a few other things like that.
That being said, if you are comfortable working off the command line, there's nothing stopping you from running the same services (except for Apple's proprietarial stuff) on a normal OS X box. For example, I use my wife's iBook with a dead screen as a server for various things on our home network.