German chocolate cake and devil's food cake have many differences. Some of the differences are the type and sweetness of chocolate used, the type of flour, the number of eggs and most noticeably, the icing with which they are topped. Devil's food cake is less sweet and denser than German chocolate cake. Many people use devil's food cake mix topped with German chocolate icing to make a quick version of the cake.
German chocolate cake is made with German sweet chocolate; devil's food cake is made with unsweetened chocolate. German sweet chocolate is dark baker's chocolate with sugar added. It did not come from Germany, but is named after the creator of the chocolate, Samuel German. Unsweetened chocolate used in devil's food cake creates a dark, chocolate cake mixed with the same amount of sugar used in German chocolate cake. Essentially, German chocolate cake is double sweetened through the extra sugar in the chocolate plus the addition of granulated sugar.
German chocolate cake is fluffier than devil's food cake. This fluffiness comes from the use of whipped egg whites before they are added to the batter. The eggs are separated, the yolks go in with the batter, then the whipped whites are folded into the batter gently to keep the air in the batter, making it a fluffier cake. Devil's food cake is a dense, moist cake that usually uses one or two less eggs than German chocolate cake. The eggs are added, without separating, to the sugar to start the batter.
The flour used in German chocolate cake is cake flour, which aids in the fluffiness of the cake. The traditional version of the recipe uses cake flour. Although some modern versions call for all-purpose flour, all-purpose flour should not be used to make a German chocolate cake. Devil's food cake uses all-purpose flour because it aids in the moist density that devil's food cake has. Cake flour is a finer grind of flour that allows cakes to rise more providing a fluffier cake.
Devil's food cake can have different icings and still be called devil's food cake. It is not associated with the type of icing with which it is topped. German chocolate cake, on the other hand, is known for its caramel-flavored icing laced with coconut and pecans. German chocolate is so well-known that you can make a devil's food cake, top it with German chocolate cake and many would argue with you that it is a German chocolate cake because of the icing alone.