it works a lot like a photo copyer. the scanner bulb lights up the image, while a digital imager, records the image and saves it onto the scanners memory, then sends it to the computer via the usb cable, or if its older a serial cable.

Consider it a copy machine that rather than printing an image on paper, it becomes a computer file.

It works just like a copy machine or fax, except it prints to an electronic file instead of paper.
-MM

Scanner works quiet similar to the technology used by Photo Copier and Fax machines.

The Scanner Lamp scans the document/picture, it then sends the data to the computer. Computer reads the data using the TWAIN (Scanner Driver) and it converts the information in to JPEG format or any other format as specified.

For Detailed information of How a scanner works click on the below link and it will give you better information.