Graphite is a giant covalent substance made entirely of carbon but it has very different properties from diamond.
In graphite, each carbon atom is bonded covalently to three other carbon atoms in a planar configuration.
Each carbon atom shares only three of its four valence electrons. The remaining valence electron is delocalised. In other words, it is not associated with a particular covalent bond location.
The planes can slide over each other because there are no covalent bonds between one layer of carbon atoms and the next one. This makes graphite a soft compound.
Because of its softness, graphite is often used as a solid lubricant to reduce the friction between two surfaces.
The delocalisation of electrons allows graphite to conduct electricity.