Whether we're talking electricity or gravity, potential differences are always defined _with respect to something_ -- where you put your zero is pretty arbitrary, and for some problems there's an obvious "right answer". For stuff to do with space, for example, we might want to put the zero of potential at the edge of the universe: at a glance, anything with positive energy therefore has enough to get there (and is "free" from any ground gravitating body) and anything with negative energy is "bound" to never get there (and typically bound into orbit around something else).
It's the same idea for electricity. Potential differences are what drives currents: at some level, that boils down to more electrons over here, more electrons over there.
The thing about the ground is that, usually, we're all on it. From a safety point of view, this matters, because we're slightly conductive bags of meat that rely upon electricity in certain bits to live. Bugger up the electrics, stop us living. As we always have one foot on the ground, the potential of the earth is a good reference point to chose, and it's the convention that for electrical thingywhatists, _it's_ the zero of potential, i.e. 0 V. By definition, therefore, electricity is anything that develops a potential difference with respect to this -- it will generate an electric field to move charges around, and the charges feel a force as a consequence.
Conveniently, unless you live in a desert soil is itself quite conductive, and a long (typically >4 m long) rod of metal driven directly into it. Often your power company will do this for you as the niggly details about "how big", "how many" and "how wet is the soil" turn out to be quite important, and also quite complex. Concrete doesn't easily work as it's an insulator: wet, loamy soil works brilliantly. People have written whole books and PhD theses about how to deal with granite, sand, and similarly frustrating geology.
Ordinarily, in most power distribution systems around the world, the flow of charges -- the electrical current -- doesn't actually use this ground all that much. You can think of it coming in via live and leaving via neutral, if you want. The permanent earth is a protective feature to ensure that a very large current _could_ flow to the ground if it got the opportunity to, and _not_ flow through you. This is a safety device in disguise, as it'd trip a circuit breaker and protect the wires in your house from catching fire.
Mains electricity is a bit different: because of how generators work, it turns out to be easier to generate (pun not intended) three _different_ live wires all at the same time, called L1, L2, L3 or three different phases. They are all AC voltages, developed with respect to each other, 120ยบ out of phase. At the substation that transforms these to lower voltages, the central tap of the transformer is hammered into the ground, and your neutral wire -- which is also connected to that central tap of the transformer -- therefore is about ground too. This provides a nice, handy, readily accessible common reference that both prevents you frying and lets your electric hob continue doing so at the same time.
It's the same idea for electricity. Potential differences are what drives currents: at some level, that boils down to more electrons over here, more electrons over there.
The thing about the ground is that, usually, we're all on it. From a safety point of view, this matters, because we're slightly conductive bags of meat that rely upon electricity in certain bits to live. Bugger up the electrics, stop us living. As we always have one foot on the ground, the potential of the earth is a good reference point to chose, and it's the convention that for electrical thingywhatists, _it's_ the zero of potential, i.e. 0 V. By definition, therefore, electricity is anything that develops a potential difference with respect to this -- it will generate an electric field to move charges around, and the charges feel a force as a consequence.
Conveniently, unless you live in a desert soil is itself quite conductive, and a long (typically >4 m long) rod of metal driven directly into it. Often your power company will do this for you as the niggly details about "how big", "how many" and "how wet is the soil" turn out to be quite important, and also quite complex. Concrete doesn't easily work as it's an insulator: wet, loamy soil works brilliantly. People have written whole books and PhD theses about how to deal with granite, sand, and similarly frustrating geology.
Ordinarily, in most power distribution systems around the world, the flow of charges -- the electrical current -- doesn't actually use this ground all that much. You can think of it coming in via live and leaving via neutral, if you want. The permanent earth is a protective feature to ensure that a very large current _could_ flow to the ground if it got the opportunity to, and _not_ flow through you. This is a safety device in disguise, as it'd trip a circuit breaker and protect the wires in your house from catching fire.
Mains electricity is a bit different: because of how generators work, it turns out to be easier to generate (pun not intended) three _different_ live wires all at the same time, called L1, L2, L3 or three different phases. They are all AC voltages, developed with respect to each other, 120ยบ out of phase. At the substation that transforms these to lower voltages, the central tap of the transformer is hammered into the ground, and your neutral wire -- which is also connected to that central tap of the transformer -- therefore is about ground too. This provides a nice, handy, readily accessible common reference that both prevents you frying and lets your electric hob continue doing so at the same time.