Pistons are metal blocks (cylindrically shaped) that travel up and down a hollow cylinder in the engine, creating compression. The piston compresses the air and gas inside the combustion chamber. This causes combustion and the piston is forced down. These movements are called "strokes". Forged pistons are more expensive and if you're racing or using a lot of horsepower, forged is the way to go as they are much stronger than cast pistons.