Original: 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. 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.
Simplified: The Devil's food cake is an old fashioned chocolate cake made of chocolate or cocoa and baking powder.