A digital camera takes light and focuses it via the lens onto a sensor made out of silicon. It is made up of a grid of tiny photosites that are sensitive to light. Each photosite is usually called a pixel, a contraction of "picture element". There are millions of these individual pixels in the sensor of a DSLR camera.
The sensor in a digital camera has a particular fixed sensitivity to light. To achieve the correct exposure for an image, just the right amount of light must be let into the camera to hit the sensor. This is accomplished by a shutter which opens and closes for a given amount of time, and an diaphragm in the lens which changes in size to let more or less light in.
with this ebook:
Basics of photography and how to take better photo