You can place paper or objects like open books on top of the glass. Light emitted from under the glass reflects off the object being scanned and is picked up by the moving CCD array in the scan head.
Flatbeds are a popular and flexible type of scanner that come in both monochrome and colour.
Their only draw back is that they hog a lot of desktop space. Unlike a flatbed scanner, the photosensitive array in a sheetfed scanner is stationary, and the page being scanned is moved over the array. This is a good type of scanner for someone who intends to scan a lot of documents to produce editable text. Sheetfeds are available in both monochrome and colour. Although you can feed pictures or small pages into the scanner, the scanned images can get warped so flatbeds are still more flexible.