There are logically four wires involved with supplying the
	main panel with power.  Three of them will come from the utility
	pole, and a fourth (bare) wire comes from elsewhere.

 	The bare wire is connected to one or more long metal bars pounded
 	into the ground, or to a wire buried in the foundation, or sometimes
 	to the water supply pipe (has to be metal, continuous to where
	the main water pipe entering the house.  Watch out for galvanic
	action conductivity "breaks" (often between copper and iron pipe).
	This is the "grounding conductor".  It is there to make sure that
	the third prong on your outlets is connected to ground.  This wire
	normally carries no current.

	One of the other wires will be white (or black with white or
	yellow stripes, or sometimes simply black).  It is the neutral wire.
	It is connected to the "centre tap" (CEC; "center tap" in the
	NEC ;-) of the distribution transformer supplying the power.  It
	is connected to the grounding conductor in only one place (often
	inside the panel).  The neutral and ground should not be connected
	anywhere else.  Otherwise, weird and/or dangerous things may happen.

	Furthermore, there should only be one grounding system in
	a home.  Some codes require more than one grounding electrode.
	These will be connected together, or connected to the neutral
	at a common point - still one grounding system.  Adding additional
	grounding electrodes connected to other portions of the house
	wiring is unsafe and contrary to code.

	If you add a subpanel, the ground and neutral are usually
	brought as separate conductors from the main panel, and are
	not connected together in the subpanel (ie: still only one
	neutral-ground connection).  However, in some situations 
	(certain categories of separate buildings) you actually do
	have to provide a second grounding electrode - consult your
	inspector.

	The other two wires will usually be black, and are the "hot"
	wires.  They are attached to the distribution transformer as
	well.

	The two black wires are 180 degrees out of phase with each
	other.  This means if you connect something to both hot wires,
	the voltage will be 220 volts.  If you connect something to the
	white and either of the two blacks you will get 110V.

	Some panels seem to only have three wires coming into them.
	This is either because the neutral and ground are connected
	together at a different point (eg: the meter or pole) and one
	wire is doing dual-duty as both neutral and ground, or in some
	rare occasions, the service has only one hot wire (110V only
	service).