Image sensors convert images into electrical signals that your camera then turns into pixels. Those pixels then become a digital photograph.
Sensors also determine how much light should be taken in during the brief time the shutter is open. Small sensors are found in camera phones, while large sensors can be found in DSLRs. The size of the sensor determines the resolution of the picture. Small sensors produce small resolution images; large sensors create large images.
Benefits of a small image sensor
Benefits of a large image sensor