Most scanners sold today are "flatbed" scanners. Using them is just like using a photocopier-place your picture or piece of paper face down on the scanner and run the scanning software.

Resolution is measured in dots per inch (dpi) that the scanner can scan--the higher the number, the better the quality of the scanned image. You'll see specs that list the "optical" (hardware-based) resolution, and another spec that lists the "enhanced" (software interpolated) resolution. Most scanners today support resolutions from 300 to 3200 x 6400dpi.

A device called a "CCD" (Charged Coupled Device) or a "CIS" (Contact Image Sensor) inside the scanner determines the number of pixels, or optical resolution, that the scanner can actually scan. A CCD is better than a CIS, because it's much more durable, and provides better scans. CIS scanners will generally achieve 300 x 300dpi scans at best, where a CCD scanner can do much better.

Bit depth, which is also called color depth, is something else to consider when shopping for a scanner. It refers to the number of bits used to represent each pixel. The greater the bit depth, the more colors or grayscales can be represented. Most scanners today boast 48-bit capability, which offers great detail even in shadows and highlights. The more bits you have, the better the scan quality.

If you're scanning pictures for posting on a website, you should scan them at 300dpi and save them as 72dpi JPG or GIF files; anything above that resolution will take a lot longer to load when someone tries to view the page. If you're scanning photos that will be printed, you should scan them at 600dpi minimum, or they'll look grainy. Keep in mind that the higher the resolution you scan and save your images at, the more hard drive space the image will eat up. So, if you plan on doing a lot of scanning, make sure you have a huge hard drive or other storage device.

Many scanners feature "shortcut" buttons, which give you a one-touch means of scanning images directly to email or applications, as well as easy access to other functions.

One last thing to consider, for some people, is the availability of a transparency adapter. If you want to scan slides or transparencies, you'll need one of these, and they're generally only available as an option on higher-end scanners. More and more scanners also come with slide adapters built-in to the lid or as an attachment.