Spars are the long, strong, members in the wing that go from tip to root. Wings made of balsa seldom have balsa spars, the spars are normally of spruce, or basswood. There are normally two spars, one at the top side, and the other at the bottom. They are sometimes connected together with a sheet of balsa to make an 'I' beam type arrangement. The structure between the spars is called the "shear web". The shear web is supposed to have the wood grain running perpendicular to the spar, but this is seldom done in practice. In practice, when you build a wing, the wing ribs are attached to the leading edge, the spars, and the trailing edge.

Basically, if you take the thickest part of the wing, at the top there is a spar that runs the length of the wing, and at the bottom too.

This is taken from dictionary.com:
A main structural member in an airplane wing or a tail assembly that runs from tip to tip or from root to tip.

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