The manufacturers might like you to believe that cake mixes are some great mystery and that their recipes are closely guarded corporate secrets, but they're not that complicated. They are like any other baking mix in that they contain flour, sugar and the leavening needed to make the cake rise.
In addition, they contain shortening, emulsifiers, colorings and flavorings such as chocolate. The shortening makes the cake tender, the chocolate is for flavor and color, and the emulsifiers make the cake moist. Emulsifiers bind the fat and the liquid together; soy lecithin is the most common emulsifier.
Mixes also contain flavorings like vanillin, which is an artificial vanilla flavoring, thought by many to be the source of "cake mix taste," an aftertaste you get from cake mix. Yellow cake mixes suffer the most from "cake mix taste," I think, and they contain a heavy amount of artificial colorings, too. Chocolate mixes, on the other hand, are less cluttered. Undoctored, they deliver the least "cake mix taste" of all, no matter what brand.
All you need to add to a cake mix is liquid (water or buttermilk, for example), vegetable oil (this varies from 2 tablespoons to a 1/2 cup or more) and eggs. The number of eggs affects the texture and appearance of the cake. One-egg batters will yield a dry and tender cake, perfect for pressing into the bottom of a springform pan as the base for a cheesecake. Two-egg batters make dense sheet cakes, three-egg batters are just right for spongy layer cakes and four-egg batters turn into those sturdy and well formed Bundt and pound cakes.
You will, of course, want to add more to a cake mix, and that is why my books are so popular! I advocate incorporating mashed ripe bananas and other fruit, sour cream, chopped nuts, brewed coffee and a pantry of other ready ingredients into chocolate cake mix batters to turn them into your signature desserts.