The foam on top of beer is called a head. It is caused by bubbles of carbon dioxide rising to the surface. The carbon dioxide may be produced naturally by the activity of the brewers yeast, or artificially by dissolving carbon dioxide under pressure into the liquid. The density and longevity of the head will be determined by the type of starch from which the beer was fermented. Wheat starch tends to produce larger and longer lasting heads than barley starch. From Wikipedia.com