Foam cakes are mixed multiple times. First, you sift and measure the flour. Then you beat the egg whites and add salt and cream of tartar. You beat the eggs until soft peaks form. Then, sugar is added and beaten to form stiff peaks. To finish, you fold the flour mixture into the stiff peaks.
Step 2: Add salt or cream of tartar to the egg whites. Beat egg whtes until soft peaks are formed
Step 3: gradually add sugar until stiff peaks are formed.
Step 4: fold the flour into the eggs, without overmixing
This cake fits the description of a foam cake because there is no added fat. Egg whites also are the only part of the egg used.
The attached video includes a recipe and demonstration of angel food cake. Angel food cake is the best example of a foam cake.