What is the definition of Avogadro's constant and when should I use it?

Avogadro's constant is defined as the number of atoms/molecules in one mole of a substance. It's value is approximately 6.023 x 10^23.

It is often used in conjunction with the equation "moles = mass (in grams)/relative molecular mass" and "number of atoms/particles = Avogadro's constant x moles" to determine the number of atoms in a mass of a substance or element. 

You should use it for determining this, as well as remembering the definition as it is useful for understanding what a 'mole' is - one mole of a substance is 6.023 x 10^23 atoms or molecules of that substance.