Classic cake has a frosting (buttercream, powdered sugar frosting, etc.) while a coffee cake has a streusel or crumb topping. A crumb cake is very similar to coffee cake but the crumb topping is the star. The crumbs are bigger and the topping layer is thicker. This is the biggest difference between these desserts. Coffee cakes can have fruit just like cakes. The topping is what sets them apart.