Regents Physics - Types of Collisions*
When objects collide, a number of different things can happen depending on the characteristics of the colliding objects. Of course, we know that momentum is always conserved in a closed system. Imagine, though, the differences in a collision if the two objects colliding are super-bouncy balls compared to two lumps of clay. In the first case, the balls would bouncy off each other. In the second, they would stick together and become, in essence, one object. Obviously, we need more ways to characterize collisions.
Elastic collisions occur when the colliding objects bounce off of each other. This typically occurs when you have colliding objects which are very hard or bouncy. Officially, an elastic collision is one in which the sum of the kinetic energy of all the colliding objects before the event is equal to the sum of the kinetic energy of all the objects after the event. Put more simply, kinetic energy is conserved in an elastic collisions.
NOTE: There is no law of conservation of kinetic energy -- IF kinetic energy is conserved in a collision, we call it an elastic collision, but there is no physical law that requires this.
Inelastic collisions occur when two objects collide and stick together. In this kind of collision some of the initial kinetic energy is converted into other types of energy (heat, sound, etc.), therefore kinetic energy is NOT conserved in an inelastic collision.
In reality, most collisions fall somewhere between the extremes of a completely elastic collision and a completely inelastic collision.
* Although elastic and inelastic collisions are not specifically included in the Regents Physics curriculum, they are included here briefly for general knowledge and understanding.