Plums are members of the genus Prunus.
In France, fresh plums are known as prunes.
Fresh blue Italian plums are often called prune plums, although this has nothing to do with whether they are or will be dried.
The Christmas dessert often called plum pudding contains no plums. In pre-Victorian times the word "plum" was used for raisins and the now-misleading pudding name stuck.
There is an industry move to market prunes as plum raisins or dried plums in hopes of appealing to younger buyers.
Source: Ontario Tender Fruit Producers' Marketing Board.