Collisions:

Whenever two objects strike each other since they are in relative motion and their paths cross at precisely the right time, a collision is said to take place.

CONSERVATION OF MOMENTUM:

According to law of conservation of momentum, the entire momentum contained in two objects is similar after a collision as before. The characteristics of collision do not matter as long as it is an ideal system. In ideal system, there is no friction or other real-world imperfection, and the total system momentum not at all changes unless a new force or mass is introduced.

The law of conservation of momentum applies not only to the systems containing two objects or particles but also to systems contain any number of particles or objects. Though, the law holds only in a closed system, which is, a system in which the total mass stays constant, and no forces are introduced from the exterior.