Basically, if you look at the wing from a cross section point of view, the bottom of the wing will look similar or identical to the top of the wing. In a nut shell, if it's semi-symmetrical...the bottom of the wing will be slightly rounded. A fully symmetrical airfoil (found on aerobatic/advanced planes) will be fully rounded. The top and bottom of the wing will look identical, from the cross section of the wing.

A plane like the LT 40 I believe has a flat bottom airfoil. That is, the wing will have a rounded top, and flat bottom. Here's the breakdown:

Flat bottom airfoil - Most stability, least aerobatic ability
Semi Symmetrical airfoil - Still somewhat stable, somewhat aerobatic
Fully symmetrical aitfoil - least stable, goes where you point it, highly aerobatic.

This is the "cliffnotes" explanation. It might just have confused you...when you have a wing in your hand and can look it will be a lot easier to see what the differences are.

If you browse around this website...in some of the forums, there's likely a better and more scientifically correct explanation than mine.