What's the difference between a cupcake and a muffin?

I've been trying to figure that out, myself. Apparently all the muffins I love sooo much (Like cranberry orange nut with frosted glaze) are really cupcakes. They contain the same amount of sugar.

A couple of months ago I tried to make chocolate chip muffins and they were awful. I used an old recipe from my Pillsbury cookbook. NOTHING like what they sell as muffins in the bakeries.