Inertia was best explained by Sir Isaac Newton in his first law of motion. Basically, the law states that an object at rest stays at rest and an object continues its state of motion until an external force acts on it.
Here are some examples:
One's body movement to the side when a car makes a sharp turn.
Tightening of seat belts in a car when it stops quickly.
A ball rolling down a hill will continue to roll unless friction or another force stops it.
If pulled quickly, a tablecloth can be removed from underneath of dishes. The dishes have the tendency to remain still as long as the friction from the movement of the tablecloth is not too great.
Shaking a bottle of ketchup. When bringing the bottom down, the suddenly stopping it, inertia is what causes the ketchup to come out of the bottle.
If one drove a car directly into a brick wall, the car would stop because of the force exerted upon it by the wall. However, the driver requires a force to stop his body from moving, such as a seatbelt, otherwise inertia will cause his body to continue moving at the original speed until his body is acted upon by some force.