A scanner is a device that takes a picture of an image that exists outside the computer, such as a photograph or a drawing on paper. As the scanner takes the picture, it digitizes the image (breaks it up into dots that can be recreated on the computer screen with electronic signals), and send this digital information to the computer as a file. Then you can take this file of the scanned image and use it in your work.
Scanners come in several varieties. There are "hand-held" scanners that you hold in your hand and roll over a flat image. These are relatively inexpensive (around $200) and are surprisingly good. There are "flatbed" scanners that look kind of like small copy machines, where you lay the artwork on the glass. There are "video scanners" that use a video camera to capture an image, which means you can input three-dimensional objects, including your children.
A scanner also needs software to get the job done. Various software can give you different kinds of options, such as dithering patterns, resolution, file formats, etc. Although software is always supplied with the scanner.