There are 92 naturally-occuring elements, with a further 10 or so that can be manufactured. Elements can be arranged in a table called a periodic table, which groups elements with similar chemical properties together. For example, radon, xenon, krypton, argon, and neon are all in the same group of the table and share similar properties. Such groupings led to the discovery of several elements that weren't previously known to exist.
Some elemental substances are made of molecules with two or more atoms of a particular element. These include hydrogen and oxygen, both of which have molecules made of two atoms.
see also: compound.