In the UK earthing is used as a means to reduce the chance of you getting a an electric shock in the event of a fault.

Electricity will, given a chance, want to travel to the actual earth by the most direct route. If you get in it's way it will go through you and potentially cause severe injury.
By wiring your home with earth cables this allows a safer path for electricity to flow in the event of a fault.

Your home has several earthing arrangements. Do NOT tamper with them.
-Main earthing terminal Depending on your house's particular earthing arrangements this main terminal connects either back to your electricity board's earth, or in some houses connects to an earthing rod which is buried deeply in the ground(earth)

-Main equipotential bonding of metal services. Links water and gas pipes, central heating etc. to the main earthing terminal. (See also my section on domestic wiring)

-All domestic wiring circuits have an earth cable going back to earth . (as part of the "Twin and Earth" cable). These connect back to your consumer unit which connects to the main earth terminal.

-Supplementary earth bonding is used in special locations like bathrooms. Links all non electrical metal things like metal waste pipes, baths, taps etc.