A scanner is a computer based accessory that analyzes an image, such as a photograph or printed text, or an object in some cases and converts it to a digital image. The scanner must be attached to a computer, or other kind of device that is able to interpret and display the scanned image on a screen so that a user may manipulate, save and edit the image.

The most common type of scanner is called the "flatbed" scanner, or primarily the "desktop" scanner, used by businesses and in the home for scanning in mainly images or text. Some flatbed scanners are capable of scanning 3D objects and rendering them in 3D on a computer but these tend to be found in large companies who focus on 3D structure in their work. Some flatbed are able to convert a 3D scanned object into a 2D image on a computer screen whilst keeping the general quality. In fact most modern flat scanners will do this no problem.

Other types of scanners include the hand-held scanner, where the device is moved by hand over the object in question, and the drum scanner, which captures image information with photomultiplier tubes.

Another category of scanner is a rotary scanner used for high-speed document scanning. This is another kind of drum scanner, but it uses a charged coupled device (CCD) array instead of a photomultiplier.

One of the unique features of drum scanners is the ability to control sample area and aperture size independently. The sample size is the area that the scanner encoder reads to create an individual pixel. The aperture is the actual opening that allows light into the optical bench of the scanner. The ability to control aperture and sample size separately is particularly useful for smoothing film grain when scanning black and white and color negative originals.

While drum scanners are capable of scanning both reflective and transmissive artwork a good quality flatbed scanner can produce excellent scans from reflective artwork. As a result, drum scanners are rarely used to scan prints now that high quality inexpensive flatbed scanners are readily available.

A flatbed scanner is usually composed of a glass pane, under which there is a bright light which illuminates the pane, and a moving optical array, whether CCD or CIS. Colour scanners typically contain three rows (arrays) of sensors with red, green, and blue filters. Images to be scanned are placed face down on the glass and the sensor array and light source move across the pane reading the entire area.

Hand scanners are manual devices which are dragged across the surface of the image to be scanned. Scanning documents in this manner requires a steady hand, as an uneven scanning rate would produce distorted images.