With energy, we saw that you can define a system of objects, let them interact, and as long as no work is done on the system by a non-conservative force to either add energy to the system or take energy away, energy will be conserved.  This principal, known as conservation of energy, is fundamental to our understanding of the physical world.  There is a similar conservation law for momentum that is equally fundamental to our understanding of the universe.  For a system of particles, if the net impulse exerted on the system by external forces is zero, then linear momentum is conserved.  What this means is that for an isolated system, the change in the momentum of the individual particles must add up in exactly the right way to always cancel out.  As a result, we can use conservation of momentum to determine the initial or final momentum of the pieces of a system who only interaction is with each other.