What is Boyles Law?

Boyle's Law is a statement of the relationship between the pressure and volume of gasses. Specifically it states that under isothermic conditions, i.e. when temperature remains constant, the product of the pressure and volume remains constant, or

P₁ x V₁ = P₂ x V₂

where P₁ is the pressure before some change, V₁ is the volume before the change, P₂ and V₂ are the new values after the change.