What Is CCD? What Is CMOS?
Digital camera sensors consist of a number of pixels which collect photons (energy packets of light), which are converted into an electrical charge by the photodiode. In turn, this is converted into a digital value by the analog to digital converter (AD converter). This finally allows the camera to process these values into the final image.
CCD (Charge Coupled Device) sensors convert pixel measurements sequentially using circuitry surrounding the sensor. CMOS (Complementary Metal Oxide Semiconductor) sensors convert pixel measurements simultaneously, using circuitry on the sensor itself. CMOS sensors are the more popular of the two for DSLRs as they are faster and cheaper (as they are more integrated).
Color Filter Array Sensors
Most current DSLRs use this technology. A color filter array is fitted to the top of the sensor to capture red, green, and blue components of light falling on the sensor. Therefore, each pixel is only able to measure one color, and the other two colors are estimated by the sensor based on the surrounding pixels. This can affect image quality slightly, but it is hardly noticeable on today's high resolution cameras.
Human eyes are sensitive to the three primary colors -- red, green, and blue. Other colors are worked out by a combination of the primary colors. In film photography, the different primary colors exposed the corresponding chemical layer of film. In a similar fashion, Foveon sensors have three sensor layers, which measure one primary color each. An image is produced by combining these three layers to produce a mosaic of square tiles. This is still fairly new technology, and it is currently only being used by Sigma.