The resistance of a body to a change of velocity. Alternate Explanation: This is a phenomenon that causes a body to resist an increase or decrease in speed, including acceleration from a stop, or stopping. The greater the weight of the object, the greater it's inertia will be.
Braking: A car is a good example to use for the explanation of inertia. When you try to stop or suddenly slow down a car, and especially from a high speed, the body of the vehicle is actually pulled forward and doesn't readily stop. Inertia is why the occupants and groceries in a vehicle are also thrown forward when slowing down or stopping.
The higher the speed you travel at, the greater the inertia will be, and the greater the force with which you, the vehicle, and everything it contains will be thrown forward.
Despite the fact that inertia causes the vehicle to continue moving forward, it cannot do this forever because of multiple forces and phenomena that force it to stop eventually. These phenomena and forces are gravity, friction, and drag. If these didn't exist, inertia would cause the vehicle to move forever
Acceleration: I will use the same car as an example. When you accelerate your vehicle from a stop, inertia makes that more difficult by "pulling back" on it, which is why a large amount of power, and hence fuel is required to accelerate a vehicle from a stop. This is why you are pinned back to the seat if you accelerate at a high speed.
Inertia is not always a bad thing!
It facilitates flywheel energy storage. The resistance of flywheels to stop (inertia) is what keeps them spinning, and they certainly need as much of that as they can get, so they can spin for the longest possible time.
Aerodynamic drag is a phenomenon in which air pushes back on objects that are moving. Automobiles, for example, have to move with enough force to overcome this with the use of more mechanical power. In other words, the engine has to work harder and burn more fuel to overcome drag. Vacuums can cause a similar phenomenon.
When a vehicle is travelling, it leaves a space behind it, which creates a vacuum which is then filled with the air surrounding it. Vehicles literally pull air behind them as they move, and energy is required to do this.
Gases in general cause drag, and fluids such as water will as well. Drag is actually much more severe under water.
Drag isn't always a bad thing!
While it impedes the movement of automobiles, it is responsible for moving turbines (for example: steam turbines and wind turbines).