What makes this complicated is that different brands of mouthpiece makers use different labeling methods for these characteristics. Generally speaking though, you can make the following deductions:
This is the distance from the tip of the reed to the tip of the mouthpiece (when a reed is in place). The wider the tip opening (or higher the number), the brighter the sound (think jazzy). It is also harder to play (reed has to travel farther to vibrate), but it can help you play louder without the reed choking.
Facing Lengths: The facing is the length of the reed that is not touching the mouthpiece (again, when it's in place). The longer the facing, the higher the amount of the reed is vibrating when you play. This means it may require more muscle to play with a long facing, but it will produce a fuller sound (better tone quality).
Chamber Sizes: The chamber size the how hollow the mouthpiece is. The wider (or hollower) the chamber size, the warmer and fuller the sound - but it can also sound dull if it is too wide. The narrower, the brighter and crisper.
The reason there is so much on mouthpiece specifications is because it is the most important part of your instrument (aside from the reed itself). I've seen many people make huge improvements simply by investing in a quality mouthpiece. Small changes to each of these variables can help you attain the particular tone you're looking for.
Which to Choose?
Many of the most commonly recommended mouthpieces have a medium facing. Most of them also have a medium or close tip opening (for classical playing). Chamber sizes are generally not advertised, but are usually in this medium zone as well. The moral of the story? Choose medium, medium, medium. But the best way to find a mouthpiece that's right for you is to go to a music store and ask to play test what they have/recommend.