Tin foil was historically made out of tin, but very often when we use the term "tin foil" today we actually mean aluminum foil. Tin was replaced by aluminum because aluminum foil is cheaper and more durable (because aluminum is more malleable than tin).

A foil is a very thin sheet of material. Any material that is very malleable can be made into a foil (a malleable material can be hammered or worked into a shape without breaking). Metals have the property of being malleable so foils are very often made out of metal. For example gold foil (also called gold leaf) is often used in art, decorating, and even sometimes in Science.