O.K., Pardon my noob question, but what do the numbers and letters of an airfoil mean? An example would be NACA 2021. Is there some kind of code, or are they just file numbers of some sort?

As far as I know (and I may very well be wrong) it just helps the designer keep thier different designs seperated. Just whatever they feel like naming them.

Additional information @ http://www.ae.uiuc.edu/m-selig/ads/aircraft.html

Just like Lomcevak Duck said, naming and numbering are just a way to separate different airfoils. The system used in naming these depends entirely on the designer. However, there are a few families that have meaning behind the naming. Your example of a NACA (which was the government agency before the current NASA) 4-digit airfoil is one of these families. The first number described the maximum camber in a percent of the chord length, the second number indicated the location of the maximum camber in terms of the chord length and the final number was the maximum thickness. So, if we had a NACA 4412 airfoil, it would mean that it had a 4% maximum camber located at 40% down the chord length and 12% maximum thickness. NACA produced hundreds of airfoils based on this system and then expaned to a 5-digit (NACA 23015), 6-digit and lesser known 7-digit systems. A good book to learn all about this is "Theory of Wing Sections" by Abbot.

Newer families of airfoils have the location of boundary layer transistion and all kinds of neat stuff encoded into the number of the airfoil.

They are just like shoe sizes - really and mean about as much in many cases -
For model planes you could select a full page of these identifications and EXACTLY reproduce them for say--a 1000 square inch 10 lb aerobatic model ---then do a blind test and likely never find a farthings worth of difference in performance .
However for a specific setup at a specific FIXED speed (specific task) ONE of these (if you can really find and fly it) will show up as best .
I am often accused of making light of all these "numbers"
Not so --it is just that for the most part they apply to situations most model sport flyers will never see.

So by now you should be able to figure out that the NACA 2021 doesn't make any sense. It would be a 2% camber located at 0% of the chord and 21% thick.

There's a few that really do make sense. For example the Helmut Quabek series of model glider airfoils had names that reflected their camber and thickness values. On the other hand Eppler, Selig and Martin Hepperle numbers appear to just be a series number and have nothing to do with the airfoil parameters.

thanks guys, I was afraid that was going to be the case.

What you we're told is correct. the minimum digit in the second column for a NACA 4 digit airfoil is 2, example N2215, This is according to my NACA airfoil generator software.