[Byrne]

Why is this?  All the explanations I've come across are somewhat difficult to understand (involving pi bonds and unhybridized p orbitals), and I was wondering if anyone could provide an easier to understand explanation.

[Albert]

Graphite is a good electrical conductor because of delocalasized pi electronic density on its planar surface.

Sorry, but I don't think there can be an easier and correct explaination for this phenomenon.

[Yggdrasil]

Graphite consits of parallel sheets of covalently bound carbon.  These covalent bonds are such that they are able to conduct electricity (they have a conjugated pi system).  The parallel sheets of carbon are connected by much weaker, non-covalent bonds which are unable to conduct electricity (these bonds are known as pi-pi interactions or pi-pi stacking).  This structure of graphite has a few important consequences:

1)  Since the parallel sheets of carbon in graphite are only weakly bound, they easily slide off of eachother.  This is why you can write with a pencil; when you drag the graphite tip against a piece of paper, the paper shears off the sheets of carbon, leaving a mark on the paper.

2)  Graphite conducts heat and electricity dirrectionally.  Heat and electricity can only go across a sheet but not between sheets.

[Donaldson Tan]

transfer of thermal and electrical energy utilises the delocalised electrons between the graphite sheets. Since the delocalised electrons are restricted between the graphite sheets, then electrical and thermal conduction can only occurs between the graphite sheets. This accounts for the directional behavior of thermal/electrical conduction by graphite.