A sheetfed is small, and operates when you insert a sheet of paper. A sheetfed scanner fits easily between your keyboard and monitor. Sheetfed scanners are great for scanning text quickly, but don't do photos as well as flatbed scanners.
Flatbed scanners are large, taking up a lot of space.
In addition to better quality photo and image work, they can be useful for imaging large quantities of documents if they have an automatic document feeder. Many scanners work only with your parallel port, which already runs to your printer. Some printers refuse to run behind a scanner. Consider a scanner with USB connectivity, or a serial scanner. It seems everything wants to use your single parallel port.