Short for Charge Coupled Device, CCD is an image sensor used with digital cameras that converts light into an electrical charge and processed into electrical signals. When compared to CMOS sensors, CCD requires a lot more electrical power and is much slower at converting images into digital data. However, CCD is a lot less susceptible to noise and is better in low-light situations. It can produce higher quality images with minimal noise.
CCD sensors have been used in digital cameras for much longer than CMOS sensors, but CMOS sensors are catching up in terms of capabilities. It will just be a matter of time before CMOS sensors can produce higher quality images than CCD sensors.