There are several different types of scanner, but they all work in the same way.
The most popular ones are flatbed scanners, which look a bit like photocopiers
You open up a lid, place the document you want to scan on a piece of glass and then close the lid again.
When you tell the computer to scan, the scanner moves its scanning head from side to side and progressively down the page until it's covered the entire printed area.
The scanning head contains a very bright light and a light-sensing unit called a CCD (charge-coupled display), side by side.
The CCD is the same sort of light detector that you find in a digital camera.
It detects the pattern of light being reflected into it off the printed page and produces a series of electrical signals.