If your scanner software comes with a preview feature, use it. When you want to scan a small portion of a document, the preview will let you highlight the portion you need and not scan the entire document, thus saving some time and memory. But each type of scanner has very specific purposes and functions. It has a lid that, when lifted, shows a glass plate covering wires and a light bulb. This is an overly simplistic description of a flatbed scanner, but it's relatively accurate for each make and model.
Some flatbed scanners are smaller, some are larger, but all have the general appearance described.
The flatbed scanner is called such because it has a flat plate of glass on which the item to be scanned is placed. The item does not move on the scanner. The light source moves under the glass, allowing the item to be scanned without distortion caused by movement. The flatbed scanner is relatively stationary, though usually light enough to be moved when necessary. It is important that the scanner remain still during scanning.