Earthing
Most important thing to keep in mind, is the fact that earth was historically used as an indirect connection. Instead of having a full return metal connection between the destination and the source, we used earth back then.
- Conductive capacity of materials increase if the size increases. Earth being huge, makes it a good enough conductor.
- When earth is injected with electrons coming back from the destination, it will find it ways to reach the source somehow since the source has imbalance in it's charge.
This was the method used in telegrams in the past.
Earthing and grounding are exactly the same. Just two different names in different locations. For example, US calls it grounding and UK calls it earthing.
Earthing in electric circuits today
In electrical circuits, earthing is used as of today to protect the circuit from heavy current. It doesn't have any other purpose.
It's provided with a line that has tiny resistance. The heavy current just finds this path and flows to earthing.
Earthing in electronic circuits
In electronic circuits, ground is just a reference used during design to describe the circuit.
Even though the ground isn't used in real sense since nothing is connected to earth in case of electronic circuits, they just decided to still use the name to refer to the return connections.
- Input voltage during implementation is ensured to maintain the difference between input and the ground reference.
- All return connections for the current is also implicitly covered by the ground symbol. This is a common connection.
Ground is nothing but the return route of the circuit. Drawing the return route, though, always makes the circuit look busy. Hence all return routes are represented as GND because they represent the same thing - a return route and a zero voltage reference.