It turns out that the dermis is loaded with capillaries, very small blood vessels. Capillaries satisfy the nutritional needs of the cells in the dermis, and they also help the skin perform an important cooling function in humans. The epidermis has no direct blood supply, but instead is supported and fed by the dermis. The dermis also contains nerve endings, which let you sense heat, cold, pressure, pain, etc. These help you protect yourself from burns, punctures, and so on by warning you when there is danger to your skin. Functionally speaking, therefore the dermis is where the action is.
The epidermis is your interface to the world. It has two main layers, the inner of which is living and the outer of which is dead. The dead skin cells of the outer layer are what we can actually see, and they are constantly flaking off and being replaced by new cells being pushed outward.
The living, inner layer is called the malpighian layer. It is in direct contact with the dermis, which feeds and supports it. It is here that the sun affects the skin during tanning. The malpighian layer is itself layered like this: The bottom layer, in direct contact with the dermis, is the basal layer.
