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.
Foam is created in milk for espresso based drinks by using a steam wand. A steam wand is a rod with tiny holes at the bottom that is usually made of metal and is part of an espresso machine. This rod injects steam at high pressure into the milk. This serves the dual function of heating the milk and creating the signature foam. The amount of foam created is governed by how much air is mixed into the milk. For foamier drinks, the wand should not be submerged in the milk but rather be partially exposed to the air so that air can be incorporated into the drink. Well made foam should be smooth, shiny and velvety in texture.