OSX is a Operating system derived from BSD, and thus unix...

Unbuntu can be skinned to look like and behave like OSX (I did that in about 15 minuets)

OSX in a sense is a Desktop environment, but is very resource intensive in contrast to openbox.

(If you do it right, a computer running Linux and openbox can boot to login in 15 seconds)

(Linux is a derivative of UNIX)

So basically, OSX is a way for you to have your computer do the work you want it to do.