BIOS stands for Basic Input/Output System, although the full term is used very infrequently. The system BIOS is the lowest-level software in the computer; it acts as an interface between the hardware (especially the chipset and processor) and the operating system. The BIOS provides access to the system hardware and enables the creation of the higher-level operating systems (DOS, Windows 95, etc.) that you use to run your applications. The BIOS is also responsible for allowing you to control your computer's hardware settings, for booting up the machine when you turn on the power or hit the reset button, and various other system functions.
When the BIOS boots up your computer, it first determines whether all of the attachments are in place and operational and then it loads key parts of the operating system (the OS later loads the rest of itself) into your computer's random access memory RAM from your hard disk or diskette drive.