Beaten egg whites are used for leavening (to add an airy lightness) - and are folded in
for things like angel food cakes and soufflés.
Whipping the egg whites creates millions of tiny air bubbles (a foam like Andy M. said) and when heated the air bubbles expand. Over-beating the whites will toughen them and cause them to set before they fully expand (reducing volume), and just stirring them into the rest of the batter, instead of folding them in, will deflate them - also causing a reduction in volume.
Andy M. gave you a video on folding - here is one for