I always thought they were (perhaps regionally) different labels for the same cake. While at a local fair, I noticed that there were separate categories for Devil's Food and Chocolate cake.
PracticallyEdible has a nice description of Devil's Food Cake.
Originally, Devil's Food Cake had a medium dense texture. The colour had a reddish tint that was probably caused by baking soda reacting with cocoa powder. In fact, I have an old cookbook (The Day by Day Cook Book, 1939) that contains a recipe for Red Devil's Food Cake. This recipe calls for 2 oz. unsweetened chocolate and 1 tsp. of baking soda.
Devil's food cake is generally more moist and airy than other chocolate cakes, and often uses cocoa as opposed to chocolate for the flavor as well as coffee.1 The lack of melted chocolate and the addition of coffee is typically what distinguishes a 'devil's food cake' from a chocolate cake, though some recipes call for all, resulting in an even richer chocolate flavor.