Buckle up to save your life!
The purpose of the seatbelt is to hold the passenger in place so the passenger is almost part of the car, which prevents the passenger from flying forward as the car stops abruptly or in the case of a collision.
When a car stops suddenly due to a collision with another object such as another car, a tree, pole, guardrail, etc. the car's acceleration decreases very quickly in a short period of time. This is called deceleration. Newton's Law of Inertia explains how this happens.
As the car collides with another object, the other object provides the force which changes the speed and direction. The car stops going in the direction it was moving, and in some cases it bounces back depending on how hard the force that hits it or how much momentum the car had. Also, the speed decelerates quickly due to the impact. When all this happens the passenger is not being acted upon by a force to slow him or her down. This is where the seatbelt comes into play.
As the person continues in their same direction and speed (forward and the same speed that the car was going) the seatbelt catches him or her, holding him or her back from flying through the air.
When a passenger does not wear a seatbelt, a force will still have to act on the person in order to slow them down. This force will come from the dashboard or windshied. However the person suffers injuries in this case.
A seatbelt has two parts. The first part rests over the passenger's pelvis and the second part rests over the shoulder and across the chest. When the car stops abruptly the seatbelt applies the stopping force across a large section of the body so the damage is reduced.
Seatbelts are designed so that the machinery behind them tightens up the belt to hold the passenger in place when the car decelerates and or stops quickly. Therefore remember to always buckle up your seatbelt to avoid injuries and damages to yourself in case of unexpected deceleration and stops.