What is the difference between stress and strain?

Stress is calculated as the pressure applied per unit area. So stress is essentially the force applied to an object divided by the cross sectional area of where the force is being applied to.

Strain is calculated as the change in  parameter value (length, height or angle) as it experiences stress. It is unitless. So strain is essentially the deformation that occurs due to the stress that is applied.

Young's Modulus is a mechanical property that is computed by dividing stress over strain. It is typically expressed in GPa (giga Pascals)