is a specific kind of: plum, plum tree.

is a broader term that covers: Alleghany plum, Allegheny plum, Prunus alleghaniensis, sloe | American red plum, August plum, goose plum, Prunus americana | chickasaw plum, hog plum, hog plum bush, Prunus angustifolia | beach plum, beach plum bush, Prunus maritima | evergreen cherry, holly-leaf cherry, holly-leaved cherry, islay, Prunus ilicifolia.