The main function of the BIOS (Basic Input Output System) is to boot up an operating system on a PC. It does this by first identifying and testing all hardware components (including the hard disk and floppy drives), and then by locating a valid operating system to which it can transfer control.
The BIOS is distinct from the CMOS (Complimentary Metal Oxide Semiconductor), the latter of which, in simple terms, is the physical storage device for the BIOS set-up routines.
BIOS has become more peripheral, with the development of 32-bit and later architectures. Whereas early 16-bit DOS systems relied on BIOS for many basic functions, the high performance operating systems of today tend to interact directly with the hardware via integrated software device drivers.