Basic Input/Output System

BIOS Stands for Basic Input Output System, and is the internal software that initiates the booting process for a given PC. The BIOS itself holds all the default values for it's variable's. 
BIOS is an Acronym for Basic Input/Output System, the built-in software that determines what a computer can do without accessing programs from a disk. On PCs, the BIOS contains all the code required to control the Keyboard, Display Adaptor, Disk Drives, External Ports, and a number of miscellaneous functions.
The BIOS is typically placed in a ROM chip that comes with the computer (it is often called a ROM BIOS). This ensures that the BIOS will always be available and will not be damaged by disk write failures, as might be the case with Magnetic, or other Non-Volatile Storage Mediums. It also makes it possible for a computer to completely boot itself, perform a self-diagnostic, and pass complete control of the PC over to an Operating System. Because RAM is faster than ROM, though, many computer manufacturers design systems so that the BIOS is copied from ROM to RAM each time the computer is booted. This is known as BIOS Shadowing.
Most new PCs have a Flash BIOS, which means that the BIOS has been recorded on a flash memory chip, which can be updated if necessary. It is normally housed on an EEPROM Chip, which can be erased, and written only when specifically programmed to do so by a specific application, normally used when the system is booted from a Floppy Disk, without the main Operating System loaded, which might interfere with the Flash operation.
The PC BIOS is fairly standardized, so all PCs are similar at this level (although there are different BIOS versions). Additional DOS functions are usually added through software modules. This means you can upgrade to a newer version of DOS without changing the BIOS.
A PC BIOS that can handle Plug and Play (PnP) devices is known as a PnP BIOS, or a PnP-aware BIOS. These BIOSes are always implemented with flash memory rather than ROM. 
User-Configurable BIOS settings (Such as IRQ Maps, time, date, hard drive details, etc.) are stored in the CMOS. The BIOS itself holds all the default values for it's variable's, so when the CMOS is cleared, default values are automatically loaded.