On the simplest level, a scanner is a device that converts light (which we see when we look at something) into 0s and 1s (which computers use to perform operations). Put another way, scanners convert analog data into digital information.

To accomplish that trick, scanners use small electronic components (called CCDs, PMTs, or CISs) as their "eyes." These eyes record how much light is reflected off the item being scanned, and report that information to the computer.

To capture a whole image, the scanner divides the image into a grid, and uses a row of eyes (called the scanning head) to record how much light is reflected in each location. Once the computer has all that data, it builds a file that represents the image in digital form.

Each of the cells in the grid is called a picture element, or pixel. Scanners differ primarily in how many pixels they can measure (their resolution) and in how the actual scanning process takes place.