30 - diameter (usually).  Either case or armature, depending on the mood of the motor manufacturer.
20 - length (usually).  See above.
12 - number of turns (usually).

They generally only have meaning when comparing one motor from a particular manufacturer with another motor from that same manufacturer, unless you happen to know that manufacturer A uses the same nomenclature as manufacturer B.  (And there's so many other variables that the turns count really only means something on two motors that are otherwise identical, only wound with a different number of turns of wire).

You figure what motor to use by first weighing your airplane, then estimating motor/battery/ESC weight and adding it on, then figuring on about 11 watts/ounce of full-up weight.  Then you look at the power rating for the motor (if you can trust the manufacturer), or look at the motor weight (folks have noted that there's a fairly constant motor power/weight ratio -- I can't remember it off hand), or look at the weight of motor required for RC 3-D flying (if you're going to fly CPLA).

Plus all the obvious factors: a high quality motor will last longer and take more abuse, a cheapo motor won't.  There's all sorts of factors that come into play, from the materials used to make the armature laminations to the bearings to the mechanical structure to the way that the wires are dressed.