BIOS

Basic Input/Output System

The software (firmware) that runs (executes) when a computer starts.
The first thing that happens is POST (Power-On Self-Test).
Normally, after that the OS starts up, whatever has been installed.
To control what devices the computer tries to boot from, in what order, requires changing the BIOS settings.
Each type of computer has its own secret-handshake key sequence to access the BIOS setting controls (usually when the computer starts).

Action:
When the computer is switched on the BIOS runs a series of tests in a predetermined order and tries to establish that it has the necessary hardware required to enable an operating system to boot up (this is an old electronics term meaning 'pull yourself up by the bootstraps').

This process is similar to a person preparing to leave home in the morning, checklist:
Wallet/Purse
Watch
Shoes
Clothes
?What have I forgotten, Ah! Ha! nothing, lets go outside.

The BIOS checks motherboard, memory, drives i.e. Floppy, Hard drive, CDROMs etc, if it finds the essentials it gives a happy beep and moves on looking for a boot program, if it finds something amiss it signals it's displeasure with a series of beeps in a code that, if you know the sequences, will tell you what is most likely wrong or missing.

see Entering CMOS BIOS
http://en.wikipedia.org/wiki/BIOS∞