Used as flat and round seals. In order to provide a tight seal, they must undergo plastic deformation, which enables them to fill up any surface roughness. This is why they can only provide a tight seal on non-moving surfaces. Soft metals (soft iron, copper, aluminium) are used for this purpose (exception: piston rings).
For specific cases (e.g. involving very high temperatures), metallic seals similar to grooved ring seals and which do not undergo plastic deformation are also available. They are unable to adjust to the ripple pattern on the surface of the cylinder so do not provide an absolutely tight seal.