You can run windows on OS X via bootcamp. I recall some people use rEFIt to run linux or more than 2 oses on a mac.
Apple used to use PPC at one point (and there were ARM, MIPS, and other architectures -and you can run any OS compiled for them on those - windows might run on arm, and historically ran on alpha and others), so there was a distinct difference - modern macs apparently have a fan control chip which OS X checks for when installing/booting to prevent OS X from running on other systems.
So.. the difference? what difference? There is none. Macs are X86 based systems. PCs are x86 systems - they have mostly the same interfaces with different rates of adoption .Its all standardised, just with minor differences that would be there anyway.