Only recently, and only to be more marketable, dried prunes attained the status of dried plums. Prunes and plums are actually two different types of fruit. Prunes are European fruits that dry nicely, but are not popular as fresh fruits. Plums are Japanese fruits that are best while fresh, but do not dry well at all. Because plums have less of a sugar content, they are likely to mold before they dry.
Plum is of the genus Prunus, just like prune and the other stone fruits. Stone fruits all contain large seeds, which are known as stones. Plums are ‘clingstone’, because the flesh of the fruit clings to the stones within. Prunes are ‘freestone’. Their stones separate easily from the flesh. The most popular plums are purplish or burgundy red. Others are blueish purple, red, orange, yellow or green.
Plum trees grow fast while young, and require aggressive pruning while dormant through winter. Otherwise, they get overwhelmed with fruit, and too tall to facilitate harvest. Even semi-dwarf trees can get almost twenty feet tall. They are spectacular in prolific white bloom. Small bare root trees that are now becoming available adapt to a new garden more efficiently than larger canned trees.
Tony Tomeo 
I love plums! Fresh, certainly, but also cooked in various ways. My grandmother made a stew with short ribs, sour plums, carrots, and potatoes, called tzimmis. One of my favorites! Also I like prunes, especially stewed. And plum cake!
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Were the sour plums actually plums and not prunes? They probably were . . . since they were tart. In the Santa Clara Valley, decades ago, both were very common. Also, some were used interchangeably. Prunes were commonly eaten fresh; and plums were often used in cooking. Incidentally, sour cherries somehow became rare near the end of the Orchard Period here, so people of my generation had no concept of what they were. We grew up with pies made with sweet cherries. We thought that was what cherry pie was supposed to be, so we thought they were as good and as traditional as any other pie. Nowadays, I would not recommend it to anyone who did not grow up with it. It is rather bland.
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