BIOS is an acronym for basic input/output system, the built-in software that determines what a computer can do without accessing programs from a disk. The BIOS is an important part of any computer system. On personal computers (PCs), for example, the BIOS contains all the code required to control the keyboard, display screen, disk drives, serial communications, and a number of miscellaneous functions.
The ROM BIOS Explained
The BIOS is typically placed in a ROM chip that comes with the computer (it is often called a ROM BIOS) on the motherboard. This ensures that the BIOS will always be available and will not be damaged by disk failures. It also makes it possible for a computer to boot itself. Because RAM is faster than ROM, though, many computer manufacturers design systems so that the BIOS is copied from ROM to RAM each time the computer is booted. This is known as shadowing.
From Webopedia.