CCD sensors were first introduced in 1974 and were used in a 100x100 pixel camera. Since then, they have come a long way in image quality and produced the best image quality of all sensor types until quite recently. Without going into too much technical detail, the light that falls on each pixel is transferred from the sensor through a single output node or a few output nodes. The charges then undergo a series of processes and are sent out as an analog signal. Next, the signal is amplified and converted to a number by an A/D-converter outside the sensor. Due to this, one of the disadvantages of CCD sensors are the need for analog components that require more electronic circuitry outside the sensor. This can lead to over 100 times more power consumption than CMOS sensors, which can cause heat issues within the camera, negatively affecting image quality and the cost of the overall product. CCD sensors also require a higher data rate because there are not very many output nodes.
Despite the disadvantages of the CCD sensor, when the CMOS technology was first developed, CMOS sensors had inferior image quality and noise performance compared to CCD sensors because CCD sensors had been around for much longer, but as the technology in CMOS sensors was further developed these differences soon diminished. Some of the advantages of CMOS sensors include the ability to incorporate Amplifiers and A/D-converters in the sensor and other functions such as image stabilization and image compression on the same chip which can greatly reduce the cost. In addition, CMOS sensors don't require as much power to operate, have a faster readout, and don't need to be as large as CCD sensors to get decent image quality. On the other hand, CMOS sensors tend to suffer in low light conditions. This is because CCD sensors have a higher fill factor, or the percentage of the pixel devoted to gathering light.
Most older cameras use CCD sensors because when they were created the CMOS technology had not been developed enough to be a viable option for camera manufacturers, but as the image quality improved and the price went down, CMOS sensors took over the market and are the primary sensor in newer cameras. So the age of the camera you are in the market for will have a big impact on whether you choose CCD or CMOS. Currently, you can't go wrong with either one as they are the two best sensor technologies, but two new sensor types have been introduced in the past year that have a potential to replace the existing sensors in the market. Check my website later in 2014 for a comparison of the two new state of the art technologies and happy camera shopping!