Image Sensor
Modern-day scanners use two types of imager sensors, CCD (Charge Coupled Device) or CIS (Contact Image Sensor). CIS is a more recent technology, used primarily in cost-effective entry level scanners. CIS scanners are smaller in size and are more energy efficient than CCD models, however the trade off is a slight loss of quality.
Resolution
This spec is what can really set a scanner apart from the rest. The resolution refers to the number of pixels a scanner can capture and is often measured in dots per inch (dpi). High quality scanners can capture more information from an image then standard entry level document scanners and therefore provide greater detail and quality. High resolution scanners can also scan excellent images even at a lower quality. For example, a 600dpi resolution scan using a 1200dpi scanner can be better than scanning the same resolution on a 600dpi scanner.
In terms of optical resolution, 300 dpi is adequate for the average document scan, while 600 dpi should be good enough for high-quality scans and common photo scanning. 1200 dpi or higher is required only for highly-demanding graphics work/photo scanning and 3200 or higher for film/slides.
Colour/Bit Depth
The bit depth refers to the amount of information a scanner is capable of recording per pixel. The larger the bit depth, the more colour and grey shades the scanner will observe and in turn this results in a better quality scan. With higher quality images also comes larger file sizes as more information is stored per pixel.
Dynamic Range
Dynamic range is a measure of a scanners ability to record different tones of an image. The range is the difference between the darkest and brightest optical density that the scanner can capture. The bigger the difference, the better quality you will have.
Speed
Speed of a scanner becomes more of a factor based on the frequency of use. The more you use it, the more likely you will require greater speeds to complete your work efficiently. Scanner speed is measured in PPM (pages a minute) or IPM (images per minute).
Keep in mind that the speed of a scanner is affected by certain features and settings. For example a higher quality resolution scan will take longer, as well as scanning duplex.