Every time the computer starts it runs the BIOS. The BIOS is a bit of code that is programmed in a ROM chip at the factory. In older computers this was non-programmable. But in newer computers, the ROM chip is now referred to as Flash memory, meaning the programming can be changed with a device that connects to the chip, even while the chip in installed on the motherboard.
The BIOS contains two parts. The actual code that runs the pre-start programs is a part of the BIOS. The second part is a setup program you can go in using Setup on your computer. This is the only part of the BIOS where you can configure settings for the computer. This part of memory is stored on what is referred to as CMOS (complementary metal oxide semiconductor). You can access this part of the BIOS by hitting a key or combination of keys when you first start your computer. Some computers have a BIOS that requires the F2 key to be hit while booting the computer. Some computers require a combination of CTL, ALT, ENTER, or some other combination.
Either way you do it, once you enter the setup program, you are now able to enter configuration information for such things as specs on your hard drive, CD-ROM, how the boot process goes, setting up your network, and many other tasks. Newer computers have a BIOS that was designed to allow for password set up and power management features. This helps control the computer during and after boot up.
There are several types of BIOS out there including Phoenix, IBM, and American Megatrend, to name a few. Each one is created differently, but with the same purpose of allowing certain functions on your motherboard and in your computer to be changed when necessary.