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).
Viton® is a synthetic, high-performance fluoropolymer elastomer (rubber) commonly used in o-rings for rigorous applications. Categorized under the ASTM D1418 and ISO 1629 designation of FKM (hence, why they're also referred to as "FKM o-rings"), Viton o-rings are preferred because of their high resistance to extreme heat and brutal atmospheric conditions, as well as to oxygen, mineral oil, fuels, hydraulic fluids, aromatics and many organic solvents and chemicals.