The chips and operating systems for the Macintosh and the PC (which started with the Microsoft DOS platform but is now almost always running Windows) are completely different. There are applications such as SoftWindows that emulate a Windows system on the Macintosh (there are more programs available for the PC than Mac, so there are no programs going the other way).  For practical use, this is the most fundamental level.  It translates the user's instructions into machine language to be manipulated by the processor, and shows the result of the specified operation when it is returned by the processor.  It's this that incorporates all the modules and layers needed for convenient programming and operation, and this collection is generally known as a 'version', as in Windows XP or Mac OSX.  It's this platform, the operating system, that can get cranky when not addressed properly by the program.  Not all building blocks fit together perfectly, and this is the source of 'bugs'. You can see why programs need some fine tuning when a new operating system is published.

The operating system sets up the initial state of the electronic hardware, and effectively sits on top of the basic computer and directs the changes in its electrical state, according to the directions of the computer user.  It provides the convenient human interface and can direct optional external devices such as hard disk drives, printers or CD-ROM drives.  Each revision of the operating system will embrace more such devices, at the same time as it facilitates more types of programs.  In practice, however, it is more convenient for hardware and software manufacturers to write small additions to the operating system to 'hook' something extra into it.  These are the source of most of the bug trouble.  They are known as 'drivers' (PC), or 'extensions' (Mac).