Ground, or earth wires connect directly to the earth, providing a point of comparison to the voltage in the rest of a system, and protecting devices (and people) from insulation failures by triggering circuit breakers to cut power. Voltages significantly above that of the ground can be dangerous, causing electrical shocks to grounded people who touch them.
Ground connections also prevent static electricity from building up in some devices, and in some kinds of circuits, the ground can even function as a conductor, reducing costs.
Even many electronic devices with no physical connection to the earth have a conductor referred to as the ground. Also known as "common," they provide a unified avenue for current to return from different places on a circuit.