In graphite, carbon is sp2 hybridized. Each carbon is thus linked to three other carbon atoms forming hexagonal rings. Since only three electrons of each carbon are used in making hexagonal ring, fourth electron of each carbon is free to move.This makes graphite a good conductor of heat and electricity. Further graphite has a two dimensional sheet like structure. These various sheets are held together by van der Waal's force of attraction which makes it difficult to melt. Further due to theseweak forces of attraction, one layer can slip over the other. Which makes graphite soft and a good lubricating agent.