A Scanner is a device which allows you to capture photographs or drawings or text from tangible sources (paper or slides etc.) in electronic form. Scanners work by detecting differences in brightness of reflections from an object or image employing light sensors. These light sensors are organised in an array across whole width that is scannable. This packing concludes the resolution and details which can be scanned. 

Scanners come in several types:  Drum Scanners, Flatbed Scanners, Hand Scanners and Video Scanners. Drum Scanners employs a rotating drum to scan loose paper sheets. Flatbed scanners have movable sensors to scan images situated on a flat glass tray.

These are the most costly kind of scanners.  Hand held Scanners are cheapest and most portable.  

They are convenient for many applications however are small in size and require good hand control for high quality scanning.  Video Scanners employ Video technology and Video cameras in place of Scanning technology.  Possibly they can give high resolutions, scanners in economical range give poor resolutions.

Figure: Scanners