Oil seals, also known as grease seals, are rings that have a moveable lip that helps keep liquids from leaking out or dirt from coming in by rubbing against a housing or shaft.  To properly install it, the lip of the seal should be pointing in the direction of the medium which is being contained.  Some grease seals or oil seals include a spring to help ensure that the lip makes contact with the shaft.  The way the seal is oriented and its direction is an important consideration.  The different orientations and directions could be internal, also known as a rod seal, external, also known as a piston seal, a symmetric seal, or an axial seal.  A rod seals would be a radial seals or a shaft seal.  This seal is fitted directly into the housing bore with the shaft contained inside the sealing lip.