The most general definition of foam is a substance that is formed by trapping many gas bubbles in a liquid or solid. It can also refer to anything that is analogous to such a phenomenon, such as quantum foam. Often the term is used in reference to polyurethane foam (foam rubber), XPS foam, Styrofoam or some other manufactured foam. It can be considered a type of colloid.(Copy from Wikipedia:Foam)

To create a foam, you need to start with a liquid viscous enough to hold some shape. Water alone will not do this. Try taking room-temperature egg whites and whisk them in a large bowl. Do this vigorously until you produce... (ta daa!) foam.