Phase 0 is the phase of a stable resting potential, when the
cells are polarized and in an excitable state awaiting a stimulus
which will cause 
depolarization. When a stimulus
above the
threshold potential strikes the cell the cell begins to
depolarize. Sodium ions rush into the cell causing the
electrochemical difference potential between the inside and
outside of the cell to race toward zero.
During Phase 1, the depolarization phase,
the
electrochemical voltage change is so rapid that the voltage
overshoots the zero potential and tops out around 
+20mV.
Phase 1
is a very short phase where the potential difference comes to
rest near 0mV.
Phase 2, referred to as the plateau phase, is where the
transmembrane action potential is maintained near 0mV by the
infusion of 
calcium ions. The cell is in a
depolarized state and
restoration of the resting membrane potential is beginning to
take place. At the end of this phase, the cell begins to
repolarize.
Phase 3 is known as the 
rapid repolarization phase. The
cell
restores itself to the original polarized state of -90mV.