CMOS (Complementary Metal-Oxide Semiconductor) is a technology used in fabricating integrated circuit chips. But in the context of photography, it is one of the two alternative technologies used in digital camera image sensors; the other being CCDs.
Early digital cameras tended to use CCD sensors, as this was the more mature technology. The evolution of CMOS sensors had a steeper learning curve, and originally their use was confined to lower-cost products where image quality was not an overriding concern. However Canon and Sony refined CMOS sensors through several generations; today (2011) when comparing sensors of equal size, CMOS sensors generally exceed CCDs in image quality measures.
Typically a CMOS design uses active circuitry placed on directly on the chip itself, which can speed image readout and help with noise reduction. The added circuit complexity does increase the up-front costs to the chip developer; thus we see sensor manufacturer Sony trying to recoup some of those costs by selling sensor chips to its nominal competitors in the camera market: Pentax, Nikon, Leica, etc.
As HD video has increasingly been seen as a must-have bullet point in digital camera sales, the speed advantage of CMOS chips has helped make them the default choice in new camera designs.