O-rings are one of the most common seals used in machine design because they are inexpensive, easy to make, reliable and have simple mounting requirements. They can seal tens of megapascals (thousands of psi) of pressure.
Whether a plastic or rubber seal, metric o-rings come in two main forms: static or dynamic. A static seal is where the ring remains stationary and is used simply for containing pressure or maintaining a vacuum. Dynamic seals can be either reciprocating (for use in pistons and cylinders) or rotating (for applications in which a shaft rotates inside a housing).