WHBM's examples illustrate the difference between an earthed (grounded) system and one that uses an earth return.
Most electrical systems have one point tied to earth/ground/frame for the purpose of safety. This keeps system voltages within design limits with respect to surrounding structure to protect personnel, allow for fault detection equipment to operate, limit certain types of electromagnetic interference and prevent insulation breakdown. In some systems, the grounding connection is made through a resistance to limit fault currents. However, many of these systems provide a dedicated return conductor (neutral) to complete the circuit. So, under normal circumstances, no current returns through the earth or frame.
In some cases (automotive and aircraft as examples), the earth or frame is actually utilized as the return conductor. This is done to save weight and cost, but it has a few down sides. Corrosion due to electrolysis, the difficulty in detecting certain types of faults and the larger loop area of the circuit (resulting in higher inductance, voltage drop and EMI problems) being a few of them.
