What is the difference between an elastic and inelastic collision

In all collisions momentum is conserved. However in elastic collision kinetic energy is also conserved, molecules colliding is an elastic collision.If kinetic energy is not conserved that then the collision is inelastic, two cars colliding head on is an example of an inelastic collision.You need to calculate the total kinetic energy before and after to determine is a collision is elastic or inelastic.