Nothing more than the Windows operating system is based on the DOS programing language while Mac is based on Unix.
What? There's no such thing as a "DOS programming language". And in fact, both the Windows NT kernel and Mac OS X Mach kernel are written in the same language.
On a high level, since Windows has the most market share, it's the most compatible OS in the world for the everyman use. In regards to the UI as a whole, Windows gives each application its own window to the fullest extent (I forget the technical term for this). But in short, the best way to describe Windows is being a jack of all trades OS, at least, out of the box.
Mac OS X however, is under a tighter control with Apple. This sacrifices flexibility in favor of a (Your millage may very) "better user experience". i.e., things should be more consistent and less prone to "WTFs". The UI as a whole is a document based approach. That is, you can think of the entire OS as a window, and the context changes based on which application has focus. The only cavaet with Apple is that they tend to stretch the truth a little (the "hundreds of 'new' features to the OS") , to outright lie (there are no malware for Macs, therefore, it's the 'safest' OS).
All in all, the overall differences is what you intend to do with it. Certain applications run on certain OS's, certain hardware only have drives for certain OS's. But generally, if you want more flexibility, use Windows. Linux would be the second considered option. A handful of applications you can run in the world run on Mac OS.