Typically used to retain oil within a case or casting where a shaft passes through the case (e.g. engine output shaft or fork seals) or to keep fluid out (e.g. on wheel spindles). Types of seal varying based on whether the shaft is moving vertically or rotating and the environment it works in. There are lots of good guides to oil seals and their application - often you can buy oil seals online with just the dimensions (we generally use simply bearings), but keep in mind what the original manufacturer specced the seal to do, for example seals designed to seal the gear shift shaft wont need to be as high a specification as those required to seal the crank on a two-stroke.
Also: Sealing ring (Husqvarna 1972), Seal ring (Ducati)