Foam is an important aspect of coffee preparation, and it determines, or validate quality in some coffee drinks.
A foam is a substance that is formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the regions of gas.
When steaming and frothing milk for espresso based beverages, such as latte, or cappuccino, foam is artificially created in milk, to give the beverage the specific texture. On the other hand, the layer of foam on top of the espresso is obtained as part of the brewing process. This layer is called crema, and it is one of the signs of a correctly brewed espresso.