Prunes do not refer only to dry plums. Prunes are also fresh plums such as Italian prunes (plums). There is little difference between plums and prunes. One difference is that the pit of the prune separates from the flesh much easier then it does on a plum.
Most prunes are grown specifically for drying, with the exception of the Italian prune. Maybe that is why we kind of associate prunes with being dried fruit. The Italian prune has purple flesh, and the shape of the prune is elongated.