CCD Cameras are very sensitive digital cameras. They are built around a Charge-Coupled Device (or CCD) that can detect photons (packets of light) falling into the millions of tiny buckets (or pixels) on its surface and then manipulate them so that they can be read, stored and used to reconstruct the image that the camera was looking at
In other words, they can produce digital images. These digital images can then be transferred easily around the world using the Internet and processed using special astronomical data reduction software.
The electronic chips inside CCD cameras are very much like those in video cameras and small digital cameras. However, to make them more sensitive, they have to be kept very cold - usually below -100°C!
CCD cameras only measure the brightness of an object, not its colour (hence why any single colour image from the Liverpool Telescope is black and white, the intensity of the light is coloured afterwards!), so special coloured filters are used to distinguish between the separate colours e.g. R,V,B, H-alpha.
In astronomy, CCD cameras are preferential to cameras that have other types of image sensors, as the method in which they convert the photons into an electrical signal is special in such a way that it creates high-quality, low-noise images (a high signal-to-noise ratio). This is extremely important in astronomy, as astronomers wish to eradicate any source of noise as much as possible in order to analyse the data from the observed source accurately without any interfering signals from objects that aren't the object being observed (although getting rid of noise completely is an impossible task). In this sense they are preferential to CMOS (complementary metal-oxide semiconductor) cameras as they convert the photons to an electrical signal more proportionally so that there is minor data-loss. The ratio of the photon-electron conversion is called quantum efficiency.
CCDs also tend to have more pixels per square millimetre on their 2D photon-capturing array (that is, for the same amount of area on a CMOS and CCD, the CCD will have more pixels able to capture photons). The charge (photons) from each cell is then systematically carried away to a charge amplifier – a device which turns the charge into a voltage and thus reproduces an electrical signal of the image captured by the CCD grid.
To picture the process of how a CCD reads out more easily, look at the example below using buckets of water:
So the pixels aren't read out all at the same time as having a device to do so behind each pixel would not be very cost or resource-effective. What the CCD actually does is shifts the charge from every pixel along one row and then reads out one row by sending the charge information down to the readout register. This then repeats for each row and the image that we see onscreen afterwards is a mapped out digital reconstruction.