Tubes - What do the letters mean

I'm new to tubes, so I'm trying to learn as much as possible. I'm trying to find out what the extra letters on the tube model number mean, i.e., 12AX7WA. How is the 12AX7WA different from the 12AX7 and the 12AX7A?
Tbone, the last issue of Vacuum Tube Valley focused on 12AX7's. This may be of interest to you and help answer your specific questions.
Tbone: I usually go to the Audio Asylum tube forum (which has a full search feature) to research tubes that I am not familiar with. The combined body of knowledge of the members (or inmates as they refer to themselves) is quite impressive. I have some 5687's with the WA added at the end, but have never looked into it. I have learned to judge each signal tube based on its sonic qualities in my setup and label and store them accordingly for future use. I have some cheap RCA black plate 5687's that blow away the rather pricey Sylvania Gold Brand ones that I also use. I have also not found any rhyme or reason (brand and vintage) as far as sound quality with 6922 tubes either as long as they are not new production Sovteks which all sound kind of lame. Each brand has a general signature sound, but there is a lot of variation to my ears between like tubes that test the same. I run a signal tube for 2-3 days before making notations and moving on to the next one. I also audition them with both JJ and Svetlana output tubes to see whether they work best with a bright (JJ) or soft (Svetlana) setup. The synergy between the signal and output tubes can make or break the overall sound, if you use both in the same application. I plan on using the best sounding combinations first while I still have my hearing. Sorry that this does not directly answer your question, but thought you might be interested in what I feel is a sound approach to procuring and stockpiling tubes for future use and this method may open up the purchase of less sought after tubes (less expensive) that will sing in your system just the same.
Jim and Dekay, thanks for the advice; I will take both of you suggestions and see if I can make sense of this business. So far, it's a great learning experience!
Dekay, thanks again for another tip. I was not aware of the search feature at Audio Asylum. I'm heading to "Out Of The Closet" tomorrow, but was able to pick up 20 virgin jewel boxes from a friend of mine in Culver City. His shop is next door to a fullfillment house and they happened to have a bunch of corporate instructional CD's. Free, and he gave me something that fixed the small leak in the radiator of my Alfa. It was a good day.

You seem to have a good handle on tubes. I just ordered Telefunken 12AX7's and RCA 6L6GC's -- both 60's vintage -- for my Mesa Baron.

Thanks again for the info...
Using the 12AX7 as an example, the 12 is the approximate filament voltage.
All AX7 tubes share similar characteristics, of which there are about 20: pinout, base size, gain, plate volts, etc.
The letters on the end indicate different versions of the tube. A thru F are modifications such as an change in maximum plate voltage, usually you can substitute a higher letter for a lower, but not the other way.
The other letters like the W signify some special characteristic such as a special base material or the shape of the tube.