The BIOS (Basic Input Output System) is nothing less that the first thing that starts when you press the button on your computer. When you see the white letters appear on a black background, that is the BIOS working.
: The role of the BIOS is very essential and primitive, it's only to start the computer by waking up every parts, counting attendance and find a part that contains the software required to continue booting the PC, usually the hard drive where you installed the operating system
Administration : It is possible to access the administration of the BIOS by pressing one key at startup. As against this key is different from one computer to another. Esc, F1, F2, F10, F11, F12, Del keys are the most common.
Under the hood : BIOS is contained on the motherboard in a small chip called CMOS. When we speak of "flashing" or updating the BIOS, we talk about the operation that replaces the current BIOS in the chip with a newer BIOS using a software created by the manufacturer. That is a risky operation because if it is interrupted, the motherboard is good for recycling.
EFI its successor
: The BIOS is becoming old and will probably be replaced by EFI in future computers, an improved technology adopted by Mac
since a long time but ignored by Windows
until its latest version, Windows 7.