Pencil lead is a mixture of graphite and clay.
Graphite is one form of the element carbon. Other forms of carbon are diamond - the hardest naturally occurring substance on the earth, soot, charcoal and coke. Pencils used to be made with lead, many years ago. Lead is poisonous and so sucking the end of your pencil could be quite dangerous. We now use graphite and clay because it is safer and because we can make pencils of different hardness, so that they are more useful for drawing.