Your BIOS controls the bootstrap and initialization of all devices connected to the mainboard, including all memory, processors and controller chipsets on the mainboard. The BIOS is a program that has a user interface. You can access the BIOS user interface by pressing a control sequence of characters on the keyboard at some point. Rule number one. DO NOT mess with your BIOS settings unless you know what you're doing. Change the wrong setting in the BIOS and your computer can become a very large and inefficient paperweight.
With that said, there are many settings in the BIOS that are useful to know about.
At the time this page was written, most of the latest mainboards support auto-configuration of your processor, memory and hard drives, as well as your I/O ports (COM 1 - 4, Parallel Port, USB etc.). BIOS interfaces vary between the manufacturers and even from version to version. Often, just knowing where to look for a setting can be a challenge at first. Once in the BIOS, you can configure disk drives, memory, processor, energy saver settings, I/O registers and other settings, including some built in components in the mainboard.