A Mole is merely a number. It's a number, by definition, that contains 6.02 x 10^23 things. So, instead of saying we have a billion of something or ten billion, or a sextillion of something, we say we have a Mole, or 6 followed by 23 zeros.

This is a fantastically large number, but atoms and molecules are fantastically small, so the Mole is a convenient unit of counting particles. By definition, a Mole of a gas will occupy exactly 22.4 liters. Moles are also used in calculating the molecular weight of a given quantity of a substance. By knowing the weight, we can calculate the number of particles.