A scanner is an input device. It converts a paper document that may contain photographs, drawings, or texts into a computer image file. Its internal work is like that of a copier machine. A lamp shines light onto the paper. Two or three mirrors focus the reflected light onto a lens which in turn focuses the light through filters onto sensors. The sensed reflection intensities are used to generate the computer image file. The mirrors, lens, filters, and sensors are included within the scan head and the lamp resides on top of it. Image processing software can be used to edit the image file produced. Also, optical character recognition (OCR) software can be used to convert image of text to a computer text file that can be edited using a word processor like Microsoft Word.

The scanner resolution is the amount of paper detail the scanner can detect. It is measured in dot per inch (dpi). A scanner with resolution 1200 x 2400 dpi represents each square inch of the scanned paper by a rectangle whose dimensions are 1200 dots in the X direction and 2400 dots in the Y direction. This is referred to as the native (true) resolution of the scanner in contrast to its maximum resolution which is the highest resolution supported not through the scanner hardware but through its attached software.