Plums

There are more than 140 varieties of plum sold in the United States. The plum is a drupe—a pitted fruit—related to the nectarine, peach, and apricot, but it is far more diverse than its relatives, coming in a wider range of shapes, sizes and especially skin colors. Its flavors also vary from extremely sweet to quite tart. Some plum varieties are specifically bred so that they can be dried and still retain their sweetness, and these are used for prunes. (The Wellness Encyclopedia of Food and Nutrition)
About twenty varieties dominate the commercial supply of plums and most are either Japanese or European varieties. Japanese are the nonprune plums or salicina plums. Originally from China, these plums were introduced into Japan more than 300 years ago. Most varieties have yellow or reddish flesh that is quite juicy and skin colors that range from crimson to black-red. They are also clingstone fruits—that is, their flesh clings to the pit. Santa Rosa and Red Beaut are two of the more popular varieties. Elephant heart is a large red-fleshed variety that is good for cooking. Plums are also used for their juice and often jam or a thick syrup is made out of it.
European-type plums are smaller, denser and less juicy than Japanese varieties; their skin color is always blue or purple and their pits are usually freestone, meaning they separate easily from the flesh. The flesh is a golden yellow color. These are the plums made into prunes; a few varieties are sold fresh and called fresh prunes or purple plums.
Among the better known varieties are Italian, President, Empress, Stanley, and Tragedy. Damson plums are a small-tart European-type variety used mainly for preserves.The domestic plum season extends from May through October, with Japanese types coming on the market first and peaking in August, followed by European varieties in the fall.