VOLTAGE

Voltage should be more correctly called "potential difference". It is actually the electron moving force in electricity (emf) and the potential difference is responsible for the pushing and pulling of electrons or electric current through a circuit.

Sources of electromotive force (EMF) or voltage

To produce a drift of electrons, or electric current, along a wire it is necessary that there be a difference in "pressure" or potential between the two ends of the wire. This potential difference can be produced by connecting a source of electrical potential to the ends of the wire.
As I will explain later, there is an excess of electrons at the negative terminal of a battery and a deficiency of electrons at the positive terminal, due to chemical action.
Then it can be seen that a potential difference is the result of the difference in the number of electrons between the terminals. The force or pressure due to a potential difference is termed e.m.f. or voltage.
See: electron theory
An emf also exists between two objects whenever there is a difference in the number of free electrons per unit volume of the object. If the two objects are both negative, current will flow from the more negatively charged to the less negatively charged when they are connected together. There will also be an electron flow from a less positively charged object to a more positively charged object.
The electrostatic field, i.e. the strain of the electrons trying to reach a positive charge or from a more highly negative charge is emf or voltage.
It is expressed in units called volts, short for voltage. A volt can be defined as the pressure required to force a current of one ampere through a resistance of one ohm.
To make this easier to visualise, consider the water pressure (voltage) required to pass a litre of water (current) through a copper pipe of a certain small diameter (resistance).
Also try and visualise water going through other pipes of varying diameters (smaller to larger in size). Either the water pressure required would vary or the volume delivered would vary, or both.
You have just grasped the basics of ohms law, where E = voltage; I = current in amperes and R = reistance in ohms:

This voltage can be generated in many different ways

Some examples:
Chemical (batteries) e.g. dry cell 1.5V, wet cell storage about 2.1V
Electromagnetic (generators)
Thermal (heating junctions of dis-similar metals)
Piezoelectric (mechanical vibration of certain crystals)
Photoelectric (light sensitive cells)