Complementary Metal Oxide Semiconductor
CMOS is a integrated circuit manufacturing technology. This technology can be subdivided in PMOS (if a P-type semiconductor is used, i.e. with positive charge) and in NMOS (if a N-type semiconductor is used, i.e. with negative charge). Many integrated circuits are manufactured using this technology.
In PC CMOS is synonym of configuration memory, because this memory is manufactured with this technology.
The configuration memory is used to store hardware configuration data such as hard disk type and boot sequence.
The configuration memory (or CMOS, as you wish) is a RAM memory, meaning that its contents are erased when its power is cut. In order to avoid that, CMOS it is fed by a battery, that also feeds the system real time clock (RTC).
The configuration memory contents can be altered using a software called setup, which can be called by hitting the Del key while the computer is counting memory when you turn it on.
Nowadays the configuration memory is embedded inside the motherboard chipset, in a circuit called south bridge.
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Example of a south bridge chip.