Graphite is made up of bonded carbon atoms. One carbon atom is strongly bonded to three other carbon atoms, resulting in carbon sheets. Each carbon atom is then bound weakly to two other carbon atoms, one to the sheet above it and another to the sheet below it. The strong bonds give graphite high boiling and melting points, while the weak bonds make graphite soft and flexible.
Graphite is closely related to diamonds. Both have crystalline forms. However, diamonds are one of the hardest materials known to man, while graphite is not. There is a process that turns graphite into industrial-grade diamonds - a metal catalyst and graphite are heated and pressurized together.