Essential Guide To BIOS

This guide de-mystifies and explains your BIOS.

BIOS. Where is it? What does it do? If I touch it will I break it?
This guide written by Maximum PC takes a look at the BIOS and explains most of the settings and what they do.
This is a little technical for an absolute beginner, but if you have some basic knowledge of things like motherboards and hard drives its a pretty easy read.

The article mentions flashing the BIOS - there are some standard utilities that will do this, but do check the website of the motherboard manufacturer first. Many of them have utilities for flashing the BIOS that are adapted for use on their motherboards.
There's a pretty good section on overclocking; what it is and how to do it. 
Overclocking can cause your processor to bite the dust, so some caution is advised in that area.

Ultimate BIOS Guide: Every Setting Decrypted and Explained