When you are using brake caliper pistons, you can choose from phenolic and metal alternatives. Both have their own advantages, as phenolic is great for daily use, but they cannot take too much of a load. They are made with heat treated plastic and provide great performance. They also resist corrosion much better than metal pistons, and offer improved fuel economy at a lesser cost to the customer. For everyday driving, the phenolic ones work great. But for hardcore racing, you should get the metal ones that last longer, and are harder. They also provide better performance in racing cars in the long run.
For more info, see How to Retract Piston in Rear Disc Brake