Terminology

 

Voltage

Voltage is a representation of the electric potential energy per unit charge.

If a unit of electrical charge were placed in a location, the voltage indicates the potential energy of it at that point. In other words, it is a measurement of the energy contained within an electric field, or an electric circuit, at a given point.

Ground

 

A voltmeter has two leads and allows you to measure the difference in potential (voltage) between two points.  If you try to measure the 

potential of a point by placing one lead of a voltmeter at that point, you will not get a reading.  Only differences in potential are 
meaningful. Just like gravitational potential energy.  If you lift a five pound weight three feet, you increase its potential energy by 15 ft-lb.  

This is true no matter what its potential energy is defined to be before it was raised.

It is, nonetheless, often convenient to define a certain level, perhaps floor level, to be the height at which the weight has zero potential energy. 

You are free to define the zero to be anywhere since only the change is useful.

Similarly, you can define the electrical zero of potential to be where you like.  It is usually most convenient to define the chassis of an electronic 

circuit to be at zero potential and that is then called "ground".  When power is transmitted to your home, One wire is connected to the 
earth and is considered to be at zero electrical potential and is called "ground" (for obvious reasons).

If you touch one pin of an electrical outlet and, at the same time, a metal faucet, you will feel nothing if that pin is the grounded one (faucets 

make a good electrical connection to ground through the pipe bringing water into your house).  If you then touch the other "hot"
wire and faucet, you will get a severe electric shock due to the difference in potential.  DO NOT try this at home!