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.