It might be that some more explanation might help ... or not. Earth is generally a physical connection to the earth, commonly with a fairly long copper ground rod that makes a good electrical connection to the earth and to all other grounds to the earth.
Common AC circuits have 1, 2 or 3 power wires and a neutral. It may also have a ground connection for safety. Generally the neutral and ground wires are connected together at the main circuit breaker box and this is connected to earth through a ground rod.
Houses are supplied with 240VAC or so in America where the voltage from either power wire to ground is 120VAC and the voltage between the power wires is 240VAC.
In theory (and mostly in practice) the earth can be considered as a very large wire connecting a generator with a user. If the earth and neutral have a voltage between them, measured with a meter, or show a voltage to a ground rod it means that there is an open circuit or poor connection somewhere. Any significant measured voltage between grounds or between ground and neutral is a bad thing and needs to be fixed. If the voltage between neutral or ground and the line varies that is a problem as well, indicating a poor connection.
I once saw a house where the connection between the breaker panel and ground rod was broken. There was still 240V between the power wires but the neutral was floating, so if one side had a bigger load to neutral that side had less voltage to neutral and the other side more. When the vacuum was on it was rather weak but the TV was real bright.
It is also possible that there will be a difference between the potential at different ground rods spaced a ways apart, expecially in dry ground. In industrial applications with sensitive instruments often all the neutrals on a large piece of equipment are brought to a single ground rod, rather than to the closest ground rod, so all the instrument "grounds" are the same voltage.
As an aside, this is why you need to be careful if a high voltage wire drops on your car. If you have to leave the car, like if it's on fire, you need to jump from the car so you don't touch the car and ground at the same time. It's also important to bunny hop with both feet together as you leave. The earth has some resistance so if there is a lot of current flowing to ground from a very high voltage wire, the voltage difference between your feet can be enough to injure or kill you, even though the earth is all "ground".
