"Prunus cerasus" (sour cherry, tart cherry, or dwarf cherry) is a species of "Prunus" in the subgenus "Cerasus" (cherries), native to much of Europe and southwest Asia. It is closely related to the sweet cherry ("Prunus avium") but has a fruit that is more acidic. Its sour pulp is edible.

The tree is smaller than the sweet cherry (growing to a height of 4–10 m), has twiggy branches, and its crimson-to-near-black cherries are borne upon shorter stalks. There are two main varieties (groups of cultivars) of the sour cherry: the dark-red "Morello" cherry and the lighter-red "Amarelle" cherry.

The "morello" group has fruit that is so dark-skinned it is almost a mahogany red, with dark red flesh and dark juice inside. Sometimes "morello" cherries are so dark red that they are almost black in color.

This contrasts with the other group of sour cherries, “amarelle”. Those cherries have paler skin covering flesh that is not red and gives off a juice that is almost clear.

Morellos are slighter tarter than "amarelle".

When you do see “morello cherries” referred to as though an actual cherry were named that, chances are the writer means English Morello cherries or one of several varieties of Hungarian morellos, all of which are highly prized.

Other varieties of morello cherries include Balaton, Fanal, Heimanns Konservenweishsel, Kansas Sweet, Kelleriis 16, Koroser Langenstein, Krassa Severa, Northstar, Oblacinska, Rheinische Schattenmorelle 226 and Schattenmorelle.

More Info: en.wikipedia.org