A prune is a particular type of plum that can be sold fresh or dried. Typically, fresh prunes have an easy to remove pit and fresh plums have a hard to remove pit. The most common cultivar of plum grown in the United States is the Improved French prune. For more information on prunes, visit http://www.ask.com/wiki/Prune.
Prunes are the dehydrated version of a plum. Plums are also referred to as gages. Due to the process of dehydration, prunes are shriveled and sweeter, because the sugars inside the fruit are condensed.