The two methods for producing foam cakes are:

1)  Separated Egg Method (sponge, angel food,  biscuit, roulade, chiffon, meringue, dacquoise)

2)  Whole Egg Method (genoise)

The main difference between foam cakes and butter cakes is baking powder/soda is not used for leavening (rise) in foam cakes.  Both foam cake methods use beaten eggs to give the cake volume, not a chemical leavener as in butter cakes.  When the eggs are beaten air cells form in the batter and these cells will expand in a hot oven giving the cake its volume and structure.  To make sure the beaten eggs reach their full volume, it is important they are fresh, the correct size and at room temperature. 

Separating Eggs:  Cold eggs are easier to separate but room temperature eggs give more volume when beaten.  Therefore, separate the eggs when they are cold and then cover the egg whites and yolks with plastic wrap (prevents yolks and whites from drying out) and bring to room temperature (takes about 30 minutes) before using. To separate an egg:  Have two small bowls ready.  Over one bowl, break the egg in half and allow the egg white to fall into the bowl as you carefully transfer the egg yolk back and forth between the two halves of the shell.  When the egg white has completely separated from the yolk, put the yolk in the second bowl.  If separating another egg, start with a clean (third) bowl for the new egg white, so if you get some yolk in the egg white bowl you don't contaminate the first white.  The whites will not whip properly if there is any yolk in them.  (To remove yolk; take an empty egg shell and dip it into the white where the yolk is.  The yolk will be attracted to the shell.)   Another method is to crack the egg and then place the egg in your cupped hand.  Separate your fingers and let the white run through your fingers into a bowl.  Place the yolk in another bowl. The separated egg method is the most common and some recipes use both the egg yolks and whites (sponge), while others only use the egg whites (angel food, meringue). 

Egg yolks are beaten with most (a little is used when whipping the whites) of the granulated white sugar (superfine or castor produces a finer textured cake and a smoother meringue) until the mixture is thick and lemon colored (takes about 5 minutes).  Beating creates tiny air cells which expand when the batter is placed in a hot oven. 

In a clean bowl, whip the egg whites with a little sugar to produce a meringue.  Start on low speed to break the whites up.  When a foam appears on the whites, add the cream of tartar (approximately 1/8 teaspoon for every two large egg whites).  (When whipping the whites, cream of tartar (acid) is used to stabilize them and helps the whites reach full volume.)  Gradually increase the speed to medium-high until almost stiff and then add the sugar in a steady stream until the  whites are stiff, but not dry.  (Should be thick and shiny.) 

The egg whites and flour are then gently folded into the egg yolk mixture (using a whisk or rubber spatula), taking care not the deflate the batter.  Gently pour the batter into the prepared pan (as per your recipe) and smooth using an offset spatula.  Bake as instructed in your recipe.

Angel Food Cake, Meringues, and Dacquoise use only egg whites and contain no fat (butter or egg yolks).  Chiffon Cakes use the separated egg method and contain oil and baking powder.

Genoise (French butter sponge cake or European sponge cake) uses the whole egg method.  The genoise uses whole eggs instead of separated eggs (sponge) and contains butter which makes a very flavorful and tender genoise.  It is not as sweet as the sponge cake because it contains less sugar.  Because it can be dry, a syrup is sometimes used to moisten the cake layers and this gives the genoise added flavor as well as a soft and tender crumb.  Additional flavorings can be added to a genoise batter, such as extracts, liqueurs, citrus zest, and finely ground nuts. To make a genoise put the eggs and sugar in a metal bowl over a saucepan of warm water (double boiler) and heat until the egg and sugar mixture reaches approximately 100 degrees F (40 degrees C).  (Heating the eggs and sugar first dissolves the sugar so the mixture will reach its maximum volume when whipped.)  Remove from the water bath and beat until thick and cool (about five minutes) (the batter will become lighter and paler in color as it thickens - usually starts out yellow and ends up cream colored).  The flour, and sometimes cornstarch, are gently folded in using a wire whisk or spatula and then melted butter (clarified butter is often used which adds a nice distinctive flavor) is added.  To best way to do this is to first add a little batter to the melted butter, to lighten it, and then fold this mixture back into the batter.  This will prevent deflating the batter. Pour batter into the prepared pan (buttered and floured and lined with parchment paper, which is buttered and floured) and bake (according to your recipe's directions).