What does a BIOS do for a computer system?

BIOS, in full Basic Input/Output System, computer program that is typically stored in EPROM and used by the CPU to perform start-up procedures when the computer is turned on. Its two major procedures are determining what peripheral devices (keyboard, mouse, disk drives, printers, video cards, etc.)

Is BIOS part of the operating system?

By itself, the BIOS is not an operating system. The BIOS is a small program to actually load an OS.