The law of conservation of momentum states that in any isolated system, the total momentum before an event (such as a collision) is equal to the total momentum after that event, provided no external forces act on it.
An elastic collision is a type of collision where kinetic energy is conserved before and after impact. Objects bounce off each other without any loss in energy.
An inelastic collision is a type of collision where kinetic energy is not conserved before and after impact. Some energy is lost, usually in the form of heat or deformation.