Difference Between a Cupcake and a Muffin?

Both are yummy, but there are differences. This answer is courtesy of www.differencebetween.net

Cupcake vs Muffin

While they make look similar, cupcakes and muffins actually have many differences.

Definition of Cupcake and Muffin
Cupcake is a small cake baked in a special pan that can be eaten by one person in one sitting. Apart from their size, they have all the same attributes of a cake which can include frosting and other decorations.
Muffin is a small loaf of bread baked in a shallow, round pan. Unlike traditional loaves of bread, muffins generally have more butter and sugar in them.

History of Cupcake and Muffin
Cupcakes were originally baked in small pottery cups called ramekins before muffin pans were widely available. Their name came from the fact that they were baked in an actual cup. Later, cupcakes could refer to either small or regular sized cakes whose ingredients were measured in cups, teaspoons, etc., instead of weight measurements.

Muffins were originally baked at home and were simply small bread-like additions to a meal.

Appearance of Cupcakes and Muffins
Cupcakes most of the time have frosting on the top and possibly sprinkles. Muffins may have a thin glaze or frosting drizzled over the top, but wont have a thick pile of icing. They usually have stir-ins such as fruits, nuts, or chocolate. The top of the muffin is significantly larger than the bottom of the muffin.

Summary:

Cupcakes and muffins are similar in shape but a cupcake uses a cake recipe and a muffin uses a bread recipe.

And another from www.wikianswers.com

A muffin is any quickbread (mixed by muffin mixing method & does NOT contain yeast) that is in the shape of a cupcake. A cupcake is any cake batter mixed with the straight method or creaming method made into the shape of a cup.

Cupcakes (traditionally) have frosting on top, while muffins have no frosting, but muffins can have a glaze on top. Many people think that the difference is either frosting or no frosting, or that it's "in a paper liner" or not, but that is a common mistake. Either one can be made with or without a paper liner; it's just the baker's preference.

A quickbread (or muffin) CANNOT be baked like a sheet cake or round layered cake because it won't bake properly. The same goes for a cupcake batter - a cake batter (or cupcake) cannot be baked in a loaf or bundt pan because it will not bake properly. But, since both can be baked into a smaller shape, people often think they're the same.