All matter is composed of atoms. If a substance is made of only a single type of atom, then the substance is an element. Conveniently, all of the known elements are listed on the periodic table of elements. Substances like gold, silver, oxygen, carbon, hydrogen, and helium are all elements. This means that gold is made of gold atoms, and oxygen is made of oxygen atoms. Additionally, because elements are made up of a single atom, they cannot be broken down into simpler substances; they are pure.
A substance that is made of at least two different elements that are chemically bonded together is a compound. Water is a common example. It is made of hydrogen and oxygen. If the two elements are not bonded together, then the substance is a mixture. Air is a good example of a mixture.