The plum is a stone fruit, closely related to the peach, cherry, and almond. Over 2,000 varieties of plums exist worldwide and come in a range of different colors. The skins can be red, purple, blue-black, green, or yellow and the flesh can be either yellow, green, red, pink, or orange. When dried, plums are known as prunes. (Read more about plums on World's Healthiest Foods.)