What's the difference between Alternate History, Alternate Universe, and Parallel Universe stories?

Alternate History changes the presuppositions of the canon show. Example: Clark is found by Lionel Luthor in the cornfield, rather than by the Kents. Alternate Universe splits from canon at some point during canon. Example: Nell doesn't stop Clark from kissing Lana in "Shimmer". Parallel Universe is a different simultaneous dimension. For example, rather than diverging at a single point before or during series canon, a parallel universe timeline is separate from the source universe, rather than diverging from the source universe' timeline. Think Sliders or the Star Trek mirror universe. Parallel universes co-exist and are equally valid, whereas alternate universes are self-annuling and "overwrite" the source universe. Also, alternate universes will tend to focuses on the differences brought about by an individual, whereas parallel universes involve more "global" changes such as a world in which World War I was won by the Germans, or vampires have overrun the planet.  more