Think of each dot as a tiny piece of a photo that the scanner examines. The more pieces of a photo that a scanner can detect, the more detailed it will be and the higher the resolution of the scanned image will be. The resolution on most scanners for sale today range around 600 dpi and up. If the scanner has 600 sensors per inch on the scan head, which moves across a scanned image, the resolution would be specified as 600 x 600 dpi. When buying a scanner , manufacturers may refer to two kinds of resolution.
Some scanners claim a higher dpi than is physically possible.
To figure out the best resolution to scan an image you need to know the halftone line screen resolution used for the final output on paper. A halftone is the method used to put images on paper. If you look at a newspaper picture closely, you'll discover it is made up of tiny inky dots. This is a halftone and its resolution is measured in lines per inch, not dots per inch. An image printed on a 300 dpi inkjet printer or 600 dpi laser printer would only use a line screen of 80 lpi to 105 lpi.