Consider an uncharged body like a glass rod which is given a certain charge (say a positive charge), the body acquires that charge. Now if you add more charge of the same nature on it, the charge will experience a force of repulsion due to already existing charge on it.
Therefore, some work has to be done by any external agent to overcome this force of repulsion. This work is stored up as electrostatic potential energy in the system of charges. This is analogous to the process of raising a body above the ground against the force of attraction in which work done against gravity is stored in the body as its gravitational potential energy.
Let a charge q be moved upto a distance r towards a source charge Q, the electrostatic potential energy possessed by charge q is given by
U = kQq/r
The electrostatic potential (or potential) at any point in the vicinity of a charge is defined as the amount of work done in bringing a unit positive charge from infinity to that point. If W is the work done in bringing a positive charge q from infinity to a point in the vicinity of source charge Q, the potential V at the point due to charge Q is
V = W/q
U/q = kQ/r
Electrostatic potential is a scalar quantity. It has only magnitude and no direction. Its SI unit is joule/coulomb (JC-1) or volt (V) which is given in the honour of Alessandro Volta (1745-1827), an Italian Physicist.
The potential at a point is 1 V if +1 C charge placed at that point possesses a potential energy of 1 J or the potential at a point is 1 V if 1 J of work is done in bringing 1 C of positive charge from infinity to that point.
1 volt = 1 joule / 1 coulomb