A piston fits within a cylinder wall of a piston engine, both containing and harnessing the power of combustion. The piston head is the surface of the piston that faces the combustion chamber. There are also a number of piston rings which seal off the piston walls, so combustion and oil is kept separated. The type of the engine (such as a V6, Inline-4, or V8) has a number in it which tells how many pistons are in the engine, and a word or letter noting the formation they are arranged in.
Pistons come in all sorts of sizes, but are not the only factor in determining engine displacement. An engine's displacement is found by multiplying the surface area of the piston by the stroke (distance the piston travels up and down), and then multiplying that by the total number of pistons in the engine. From this, you can tell that in two engines with the same displacement and number of cylinders, the one with smaller pistons has more stroke distance.
Piston Head Types
Piston heads come in a variety of types for different applications. Many have areas of them cut out so when the RPM is high, the valves (which become relatively slower with more RPM) don't hit the piston, which can cause major engine damage. Flat heads generate the most power because all the forces on its surface are only able to push down, instead of slightly to the side (which is the case when angles are involved), and they also have the least contact surface. They also tend to be the most peak-RPM-limiting of piston heads.
Pistons are reciprocating weight, so any effort made to lighten them (without losing structural integrity) will make an engine more lively and produce more power (power it would have produced, but was lost to extra weight). Additionally, piston heads must also be selected for different types of performance (more power or a higher peak RPM).