The human tooth is divided in two anatomical parts, only one of which is visible. The Crown, which is the part that emerges from the gum and which we are able to see when having a look into our mouth, and the Root, which is the invisible part, sinked below the gum and anchored to the bone.
Teeth differ in form and dimensions, but all of them are formed by 4 structures: we'll describe them one by one starting from the innermost towards the surface.
Familiarly called "nerve", it's the innermost part of the tooth. It's a living tissue containing blood vessels, nerves and various kinds of cells. It's the only portion of the tooth to have a real metabolism, and is the part that "hurts" if reached by a caries.
It's the rigid case enveloping the pulp for all its length, from the crown to the root of the tooth. It's a mainly calcified structure which also contains some organic components such as cells that consent it to partially modify its anatomy and, in particular, the dentin is able to create calcium deposits that protect the pulp for external injuries.
The hardest part of the tooth (and of all our body) is really a crystalized shell, a calcified cap that has the function of supporting the chewing forces. It covers the tooth only down to its neck, so it's actually the visible crown of the tooth itself, stopping at gum's height.
d) Root Cementum
It's the analogous of the enamel on the non visible part of the tooth, the root that is. It's a thin layer covering the root itself but differently from the enamel, it's much less hard. It's the portion of tooth that emerges when the gum and bone level skips lower, causing a lenghtening of the neck. It's function is to connect the tooth to the bone.