The ground and neutral should only connect at the breaker box. The neutral and hot wires have current running through them when they are suppling power to a device. In a proper system, the ground wire should not have any current running through it. As a consequence, there will be a voltage drop across the neutral wire which will not occur on the ground wire. Thus, there will be a voltage difference between neutral and ground at the load end of the wires. On short runs of heavy gauge wire it should be barely noticeable. On long runs, like a few hundred feet of extension cord, it can be several volts.
