Budget version of the valve stem seal.

As an engine wears, the clearance between the valve stem and guide increases; as this gets beyond tolerances the car will tend to smoke under deceleration (when you take your foot off the gas). Good valve stem seals will allow for greater wear before the smoke becomes intolerable. These late model seals actually have little springs around them to aid in the effort. It is possible, although tedious, to replace them without removing the cylinder head.