Prunes are not (merely) dried plums.
The common misconception that a prune is dried plum is shared by people whose only experience of prunes is with the dried form of the fruit. Prunes are their own fruit, standing out from other plums.
Technically speaking, a dried prune is a dried plum, but only because prunes are a type of plum, prunus domestica, of Family Rosaceae. Other members of genus prunus are peach, apricot, almond and cherry, as well as Japanese and American plums not of species domestica.
Plums are round, Prunes are oblong.
Prunes, varieties of which include Stanley, Italian and Brooks, are distinguished from other p. domestica varieties by their color and shape. Prunes are uniformly dark purple and distinctively oblong. Plum varieties of p. domestica, which includes the very popular Santa Rosa developed by Luther Burbank, may be yellow, green and red, as well as the common purple, and are spherical in shape. Including Asiatic species and wild American species, plums can range in size from about 1" across to 3-1/2" or more, whereas prunes have a narrower size range, about 2"-3" in length.
Prunes are notable among plum varieties because they have firm flesh and a sufficiently high level of sugar, so that they can be dried without resulting in fermentation. Hence, most people are familiar only with the dried prune, not realizing that the noble prune, while a variety of plum, has its own particular characteristics making it worthy of distinction.
In the course of researching the above write-up and clarifying my own misconception/hope that prunes were a distinct species or at least sub-species, I gleaned information from the following sources:
Sally's Place http://www.sallys-place.com
Encyclopedia Britannica http://www.britannica.com
Slider Encyclopedia http://www.slider.com/enc/index.htm