CMOS Battery gives 'backup power' to CMOS (Complementary Metal Oxide Semiconductor) of a computer to keep some parameter values, eg: RTC (Real Time Clock). These primary parameters shall be used when the system gets booted again. Many computers use CMOS memory to hold current time and date. The CMOS memory and the clock circuit that increments the time in it are powered by a small battery called RTC, so the time is correctly updated even when the computer is unplugged.
The CMOS memory can also hold the configuration parameters, such as which disk to boot from. CMOS is used because it draws so little power that the original factory installed battery often lasts several years. However, when it begins to fails computer can appear to have disease, forgetting things that it has know for years, like which hard disk to boot from.
CMOS - Acronym for Complementary Metal-Oxide Semiconductor.
RTC - Abbreviation for Real Time Clock.