I've received a lot of mail from people looking for advice on scanners.   Unfortunately, there is no easy answer, and the correct choice for you generally depends on five things:

Depending on your answers to these questions, you may be spending as little as $75 or as much as $10,000.  Let's try to narrow things down a bit...

Mechanical Resolution in Dots Per Inch (dpi). This is a measure of how many pixels are collected from your film or print when scanned.  Most 35 mm film scanners operate somewhere between 2400 dpi and 4000 dpi.  These high resolutions are needed because you are scanning a small (1" x 1.25") high-density source.  A 2400 dpi scan produces plenty of data for web use and prints up to 8" x 10" (see below).

Flatbed scanners, on the other hand, peak at between 125 dpi and 1200 dpi (depending on price of course...).  This is because most of them are designed to scan less-detailed media up to 8.5" x 14" in size.  Some flatbeds show two resolutions, e.g., 600 dpi x 1200 dpi.  The 1200 dpi dimension is somewhat interpolated and somewhat misleading.  A 600x1200 scanner is really a 600 dpi machine.  

Flatbed Design.  Flatbeds come in two flavors -- those that only scan reflective media (i.e. -  prints), and those that scan reflective media and transparencies (i.e. - prints, negatives, and slides).   Transparency scanners are further broken down into two sub-groups; those that scan transparencies without any glass in the light path, and those that scan from the regular glass bed.  Glass-free scanners are generally better in my opinion (and, you guessed it, more expensive).  The advantage is that there is no quality loss from scanning through a glass pane, and more importantly, no chance of generating various types of noise patterns that come from the transparent media being in contact with the glass surface.   Some glass-based flatbeds minimize this problem by supplying spacers to keep the media off the glass. 

Optical Density Rating. This is a measure of how much detail is extracted from dark, shadowy areas of your transparencies (or what becomes the bright area of a print from a negative).  This is also what separates consumer scanners from expensive professional scanners.  If you think about a transparency, its density can range from clear (pure white) to pure black.   With color negatives, the range is much narrower, from the faint "orange mask" color to black.  A perfect scanner would be rated at 4.0 on this logarithmic scale.  Drum scanners come close to this.  Cheap flatbed scanners are as low as 2.3, and handle dark areas very poorly.  Film scanners start at about 3.0 and run to about 3.6.  3.0 scanners work well most of the time, especially with negatives, but get very noisy with dark slides.  3.6 scanners do an excellent job with shadows,  but as you've probably guessed, generally cost a lot more.  Look at my comparison of the "old" HP Photosmart Scanner (OD 3.0) and the Nikon LS-2000 (OD 3.6) to see what I'm talking about.

Color Depth. Just as is the case with video cards, you will see ratings such as 24-bit, 30-bit, 36-bit, and 40-bit.  These determine how many unique colors your scanner can pick up.  Most scanners are at least 24-bit; some go as high as 40-bit.  You will want at least 24-bit color for high-quality photographs.   Going above 24-bit color captures extreme subtleties and tones, and gives you more flexibility (i.e. - data) when editing photos.  High bit-depth also dramatically increases file sizes.

By the way, make sure your video card is set to 24-bit color (or higher).  If it doesn't have enough video RAM for that, buy a new one! 

Bit ratings are calculated as follows:  2 "to the bit-depth" power.   1-bit images have two colors, black and white.  Not very useful.  4-Bit images, like the one above, have 16 colors; 8-bit images have 256 colors.  16-bit images have 65,536 colors -- still not enough for great skin tones or subtle shades like those in sunsets.  24-bit images have 16,777,216 colors -- more or less "true color".

Note that 24-bit color is made up of combinations of 256 shades of red, green, and blue (256x256x256).  This means that a 24-bit color scanner will only capture 256 shades of gray from a black and white negative.  This is not really enough data for displaying great skin tones.  In addition, you should know that consumer printers only print at a maximum of 24-bit color, or 256 shades of gray, making B&W portrait prints a little bit thin even if you have a high-bit source scan.

How Much Scan Data Do I Need For Prints?  Most PC displays are 800 x 600 or 1024 x 768 pixels in size, so almost any scanner that can handle transparent media  is capable of making scans big enough for a web page.  However, printing photographs is a different story altogether.  In order to avoid pixelation and loss of detail in a print, most people feel you need at least 240 dpi of resolution on the printed paper.  Assuming you want to print an 8" x 10", you'll need at least a 1,920 x 2,400 pixel scanned image.  You can get by with a little less data, but the quality will begin to suffer. 

I find that 240 dpi prints on my Epson 1200 printer are indistinguishable from custom photo lab prints in apparent resolution.  If you're working with a 35 mm slide or negative,  this means you'll need a scanner capable of at least 2000 dpi resolution to make good quality 8"x10" prints  (2000 dpi x 1 inch x 1.25 inch = 2000 x 2500 pixel scanned image).  Only dedicated 35 mm film scanners are capable of resolutions this high.

So, what sort of scanner fits your requirements and budget?  Take a look at the next page...