At thesoft peak stage, egg whites are shiny and when you lift the beater, they whites lift up and then rather gracefully curve over slightly. When folding egg whites into a batter, you generally beat them to the soft peak stage. You generally beat egg whites to the soft peak stage before adding sugar.
Thestiff peak stage occurs after sugar has been added. At this point, the lifted beater leaves tall, erect peaks, like little soldiers. (In cooking school, our instructor made us prove the stability of our egg whites by holding the inverted bowl over our heads. Fortunately, none of us ended up with egg on our faces.) These are the egg whites used to make meringue cookies and shells, such as the Mocha Cream-Filled Meringues pictured below.
Watch our how-to video on Beating Egg Whites for more tips.