Bill has given some good advice, everyone has for that matter, (I just can't see all the names while I'm in this reply box).
I just wanted to explain...in most residential services each hot lead carries 120 volts and at any given time one is 120 V+ and one is 120 V- The voltage difference between the two is 240 V. If you require 240 V, the current is carried between the two hot conductors. As long nothing else makes contact with the wires, the neutral and the ground do nothing.
Now, many 240 V appliances also use 120 V sub-circuits to run light bulbs, fan motors, and such. Those get their power by tapping into one of the 120 V lines and are connected to ground through the neutral line. In that case the neutral conductor carries the same amount of current through it as the tap into the 120 V conductor does because they make up the 120V circuit.
Even though the neutral and the ground connect to the same place, that is the earth, the neutral is designed to carry current. The ground has the capacity to carry the current but is not supposed to act as a conductor unless something goes wrong and electricity is not flowing where it is supposed to. Since all those hard working electrons are trying to get back to the earth where they originally came from, they will take the easiest path to it. So the ground conductor is supposed to provide a better path than the human body is.
IF insulation never breaks, or screws never came loose, then a ground conductor would never be needed for safety. However, a neutral conductor is always needed if you want 120 volts. Some 120V appliances have 2 prong plugs because they have double the amount of insulation and no metal parts that can become energized. They are designed that way. A 3 prong 120V appliance shouldn't shock you if the ground prong was missing IF everything is in good condition. But if not in good condition, the operator can become the path to ground, so the protection needs to be provided. Some 240V appliances do in fact require only 3 wires, 2 hots and 1 neutral (which is connected to ground). Same for them as the double insulated 120 V appliances, they were designed to be safe to use that way. But if the appliance requires a separate ground, it is best to provide it. Without it, you won't get shocked if everything is operating correctly but if anything goes wrong......
So the best advice is to follow the manufacturer's installation instructions. If you're not sure about them, an electrician's fee is cheaper than a funeral and cheaper than replacing a burnt building.
CHGuilford