CMOS stands for Complementary Metal-Oxyde Semiconductor, a technology about the production of transistors with semiconductors. Transistors are the key element of modern micro-processors and, in general, of most micro-chips. Semiconductors usually used are made of silicon and germanium, which are specifically crafted in order to control the electricity flow across them.
Main limits of the usage of semiconductors of this kind are the temperature increase related to the frequency and the physical limits reached as the production process grows smaller.