Conservation of Momentum

The concept of conservation of momentum is simple to understand by looking at the Momentum Principle.  The Momentum Principle predicts how much the momentum of a system will change based on external forces, or forces in the surroundings.  Momentum gained by the system is transferred from the surroundings, so we say the momentum is conserved.$$\Delta \vec{p}_{sys} = -\Delta \vec{p}_{surr}$$$$\Delta \vec{p}_{sys} + \Delta \vec{p}_{surr} = \vec{0}$$Basically, all force is equal and opposite.

System vs. Surroundings

In physics, a system is a collection of objects that interacts with objects making up the surroundings.  The system is observed and recorded, while the surroundings are what creates change in the system.  The momentum of a system can only be changed by interactions with its surroundings.