Image resolution of a digital camera is often limited by the camera sensor (typically a CCD or CMOS chip) that turns light into discrete signals, replacing the job of film in traditional photography. The sensor is made up of millions of “buckets” that essentially count the number of photons that strike the sensor. This means that the brighter the image at that point the larger of a value that is read for that pixel. Depending on the physical structure of the sensor a color filter array may be used which requires a demosaicing/interpolation algorithm. The number of resulting pixels in the image determines its “pixel count”. For example, an image 640×480 big would have 307,200 pixels or is approximately 307 kilopixel image; and an image 3872×2592 big would have 10,036,224 pixels or is approximately a 10 megapixel image. -Wikipedia
