All computer hardware has to work with software through an interface.
The BIOS is responsible for booting the computer by providing a basic set of instructions. Furthermore, it provides an interface to the underlying hardware for the operating system in the form of a library of interrupt handlers. Some older PC's cannot co-operate with all the modern hardware because their BIOS doesn't support that hardware. The CMOS power is supplied by a little battery, so its contents will not be lost after the PC is turned off. CMOS is the name of a technology which needs very low power so the computer's battery is not too much in use.