One mole of a compound contains Avogadro's number of molecules of that compound. Avogadro's number represents the number of atoms of carbon-12 in 12 grams of carbon-12, which is approximately 6.022 times 10^23 atoms.
The number of moles of a substance is equal to the number of grams of that substance divided by its molecular weight. The molecular weight of a compound is equal to the sum of all of its component atoms. For example, the molecular weight of water, which contains two hydrogen atoms and one oxygen atom, is approximately 18 because the molecular weight of hydrogen is approximately one and the molecular weight of oxygen is approximately 16.