Inertia is a scalar, something that only has a magnitude, and is defined as the resistance of an object to its state of motion, which also includes the state of rest. An object's mass is a measure of its inertia. Momentum is a vector, something that has a magnitude and direction. It's a measure of an object's motion and is dependent on the product of an object's mass and velocity.
The concept of inertia is seen in the tendency of massive objects to require more force to accelerate. An example would be in the collision of a car with a truck; although by Newton's third law both objects experience the same force, the car gains a greater amount of motion because it has less inertia. The concept of momentum can also be applied to the collision of the car and truck. If the car has a high enough velocity, it would continue in its original direction even after colliding with the truck. This is because its original motion, measured by its momentum, is greater than the change of motion that resulted due to its smaller inertia. The equation of momentum gives an idea of how changing an object's mass or velocity affects its momentum. Doubling an object's mass doubles its momentum, and doubling its velocity also doubles its momentum.