http://universe-review.ca/R10-11-gradient01.htm

What is membrane potential?

   A potential difference is caused by different concentration of negative and positive charged ions. Membrane potential is the electrical or voltage difference between the interior and the exterior of the cell, it ranges froom 60mV-90 mV. A membrane at rest has no stimulating pulse being received, its resting mmbrane potential is -70mV. The cell membrane is selectively permemable meaning that it denies the passage of negatively charge ions to pass across the membrane. 

http://www.winona.edu/biology/adam_ip/misc/assignmentfiles/nervous/Membrane_Potential.pdf

The exchange of sodium and potassium ions across a membrane is known as the sodium-potassium pump which for every three Na+ it pumps out two K+ ions. As the potassium ions leave the cell, they creates a more positively charged outer surface. Therefore an electric potential is created and a net positive charge and net negative charge are created outside and inside respectively. 

What is an action potential?

The figure above shows the relationship between voltage and time forming an action potential.  An action potential occurs when the there is an influx of Na+ ions and an outward flow of K+ ions in the cell membrane. 

 http://sbhepnt.physics.sunysb.edu/~rijssenbeek/PHY122_L08.pdf