Instead of films, digital cameras use a electronic device which is a sensitive on light. That device converting a light energy into electric impulses. That electric impuls after that is converted in voltage and amplified and finaly converted in digital value. Every sensor is composed with photo cells and number of that photo cells is in close relation with one of the most important caracteristic of the cameras: number of pixels. There are more type of sensor depends of type or size. 3 most important type of sensors are CCD(Charge coupled Device), CMOS(Complementary Metal Oxide Semiinductor) and Foveon. Size of sensor is very important caracteristic and sensor can be from 1/3,6 (4x3mm) to full 35mm standard (36x24mm). we are talking here just about small format (35mm) becouse that is the most expanded format in the world. i will talk later for every type of sensor.