CMOS (complementary metal oxide semiconductor) is a type of image sensor. In effect, it is the "eye" of a camera phone, digital camera, or video camera. It is a device similar to a computer chip, which senses light focused on its surface, like electronic film.
CMOS is one of two main types of image sensor. The other type is CCD.
In the past, CCD image sensors consistently provided better image quality. But now some (though not all) of the newer and better CMOS sensors match CCD quality.
CMOS sensors include basic image-processing circuitry that CCD sensors do not. This gives devices with CMOS sensors an edge when it comes to size, cost, and battery life.