Atoms and particles (Matter)

Atom is the smallest units of matter. Just read through this explaination and see whether you understand it or not:
Some substances are made of only one kind of atoms. For example, gold is made up only of gold atoms.
Some substances are made up of more than one kind of atoms. For example, water is made up of oxygen and hydrogen atoms. (please view the picture: the link below) A water particle is made of one oxygen atom and two hydrogen atoms. :))
Do you understand now?