CMOS and BIOS are often interchanged although they are different things. Think of the BIOS as the skeleton frame upon which the CMOS settings hang.
All computer memory forgets everything it holds when power to it is shut off.
Fortunately this is not the case as your CMOS always receives enough power to remember its settings, even when your computer is off. It is powered by a small lithium battery on your motherboard. Lithium batteries are designed to last years, and often outlive the usefulness of the computer they reside in. If you keep a computer long enough, you will have to replace the battery. This is usually an easy task.