What is a CMOS battery, and why does my computer need one?

All AT computers (80286 processor) or later require a small battery on the system board that provides power to the Complementary Metal Oxide Semiconductor (CMOS) chip, even while the computer is turned off. This chip contains information about the system configuration (e.g., hard disk type, floppy drive types, date and time, and the order in which the computer will look for bootable disks). The CMOS battery allows the CMOS to preserve these settings.

Also see:

• My PC won't start; what could be wrong with it?
• If my computer beeps and fails to boot, what do the beeps mean?
• What is CMOS?