The engine in common usage in cars is called the internal combustion engine. Although there have been many advances in accessories and reconfigurations of what we might call the support systems of the engine, key mechanical components have changed little since its invention and early production.
At the core of what makes an engine work are the cylinder and the piston. The cylinder is a hollow, round cavity or chamber with smooth walls. The cylinder by itself, though, would do nothing very productive. It's the piston traveling up and down inside the cylinder---with the help of a few other critical parts---that really does the work.
One of those critical parts is the crankshaft. Think of riding a bike and think of your legs as the pistons and the rods that connect the piston to the crankshaft in an engine. In fact, the part on the bicycle that the pedals attach to is called the crank and it really does exactly what a crankshaft in an engine does. Every time the pistons (your legs) move up and down they force another turn of the crankshaft.
Another of those critical parts in the engine is the cylinder head. Without the cylinder head the piston would travel up and down within the cylinder only if some external force turned the crankshaft; the engine could not run on its own.
The reason the piston can't do its job without the cylinder head is that the piston's main function is to compress a mixture of air and fuel as it travels upward inside the cylinder. In order for this compression to happen there has to be a seal at the top of the cylinder to keep the air and fuel mixture from escaping. The cylinder head forms this seal.
There are valves, usually inside the cylinder head, that open and close to allow the air and fuel mixture in and exhaust gasses out. But on this one critical stroke of the piston, the valves are closed and the air and fuel mixture is compressed. Typically there is a small space, called the combustion chamber, above the piston inside the cylinder head that contains this compressed mixture.
When the piston is at the topmost extent of its travel on this stroke and the air/fuel mixture is fully compressed, the spark plug fires. This ignites the air/fuel mixture and the result is an explosion. The valves are closed and the cylinder head is sealing off the cylinder so the force of the explosion, confined in that small, space has to go somewhere. Since the cylinder head is immovable and the cylinder itself is immovable, that leaves the piston.
The piston is designed to move downward under the force of this explosion and turn the crankshaft. The cycle repeats and as long as the seal holds---helped by a gasket that is sandwiched between the metal around the piston and the metal of the cylinder head---the engine runs.
