Xylem carries water and minerals from the roots to the leaves. Phloem carries manufactured food (sugars) from the leaves to the roots. The cambium (a watery layer only a few cells thick) is the generative layer, giving rise to both xylem and phloem.
According to Hugues Vaucher, author of Tree Bark - A Color Guide, "It would take a million pictures to cover the full variety of textures found in tree barks." Lucky for us that bark textures are relatively uniform by tree species and make a great visual marker for broad tree identification. Bark textures are divided into at least 18 types, from smooth (beech) to spiny (locust).
I've not found a key for bark and think one would be very hard to create. Only the broadest classifications can be determined using bark alone. You can very readily distinguish between an oak and a pine by looking at bark. The hard part is separating the various oak or pine species by bark alone.