Contrary to rumor, there's hardly any hardware difference anymore. Macs run Intel CPUs, nVidia/AMD GPUs and I believe Seagate HDDs. Pretty stock stuff.
There is a difference at the OS level and the BIOS (or equivalent) needs to be compatible with that. Things like EFI and GUID partition tables are the most notable low-level differences.
What is it that does not allow the MacOS to be installed on a normal PC ?
Depending on your PC, potentially nothing. The hardware may not be compatible, OS X is intended for a smaller range (since Apple has more control over what it goes on than other OSes). You may be able to find third-party drivers for common hardware, though.