MacOS (the current version being Leopard) is a derivative of Unix. The basic file hierarchy (folders and subfolders) is similar to DOS, and the tools for dealing with the file structure (the explorer) has the same drag/drop/click mechanism. One odd thing is the use of the single button mouse. One suggestion I would make is to get a two button mouse for use with the Mac coming from the PC world. The right click works similar to PC where it provides a context menu with choices based on where you are and the file-type (sorry fanboys, this one comes from windows).

Pretty much everything else is graphical based, although you do have access to the lower level unix prompt (similar to how in Windows you have access to the dos prompt).