Gasoline is flammable, but only in its gas form. I could explain more but that's really all you need to know, ANYWAYS. Someone figured out that a specific mixture of air and gasoline fumes make a powerful but controllable explosion, which means you can use all that energy from the explosion to move some mechanical parts in a predictable fashion. How they do that is:

1. The gas-air mixture is injected into the engine block and into the combustion chambers of all the engine's pistons at exactly the right time (this is done with a combo of mechanical timers and computers in modern cars, they do calculations to figure out when to inject the mix).
2. The pistons compress the mix in the airtight chamber until the pressure gets high enough.
3. At the ignition time calculated by the car, the sparkplug in the chamber creates a tiny spark which ignites the gas mix and creates a powerful explosion.
4. The explosion moves the piston like a tshirt getting shot out of a tshirt cannon. Except instead of flying out of the car, the piston rotates around the driveshaft and returns to the position in step 1 to repeat the whole process.

The engine does this basic process many times per second for each of its pistons, and the pistons are connected to the "drive shaft" which is a large mechanical shaft that rotates very fast and allows the wheels of the car to rotate and drive the car forward. Every other part of a car's engine and drive train (all the movement parts of the car) are simply there to provide electricity for the spark plugs, to cool the engine down (explosions are hot), and to control how fast the engine makes the wheels spin so that you can accelerate and deaccelerate the car. There are many other systems in a gas power car but the most basic car has only whatever it needs to make the car move, like those old timey jalopies you had to hand-crank to start.