What type of quantity is momentum?
Momentum is a measure of mass in motion.
Momentum is different that both energy and inertia.
Inertia is a measure of the mass of an object. Questions about inertia involve how much work will be required to change an object's velocity. One might also ask how an object will accelerate when a particular force is applied constantly.
Energy is a capacity for doing work. In the case of an object in motion we can judge its Kinetic Energy as a function of its mass (
While energy may be converted into other forms (heat, light, sound) when two objects collide, the total momentum will always be conserved.
One might define momentum to be a quantity which is conserved when physical objects collide.