Diamond and graphite are different forms of the element carbon. They both consist of giant covalent network structures of carbon atoms, joined together by covalent bonds. However the shape of their structures and their properties are different.
Diamond is a giant covalent substance in which:
The rigid network of carbon atoms, held together by strong covalent bonds, makes diamond very hard. This makes it useful for cutting tools, such as diamond-tipped glass cutters and oil rig drills.
Graphite is a giant covalent substance in which:
The layers in graphite can slide over each other because the forces between them are weak. This makes graphite slippery, so it is useful as a lubricant.