I will ignore three-phase and industrial wiring systems in this article, as only professionals (who already know this material) should ever deal with those, and concentrate on household wiring and appliances. Also, this will apply to ordinary, 120 vac (a.c. volts) wiring found in American households; different countries use different voltages and frequencies and standards, but some of this will apply to them as well.
First, a correction: "Positive, Negative and Ground" should read "Hot, Neutral and Ground" instead. For there to be 'positive' and 'negative', we would have to be talking about DC (direct current) systems. "Hot" is the line that carries the 120 vac (actually this value varies, and is usually closer to 115 volts). "Neutral" is the line that is held near zero volts (analogous to the 'return' wire in a DC system). "Ground" is just what it sounds like: the wire connected to actual earth ground.
Okay, so what does all that mean, and how does it help you? First understand that 'ground' is an absolute reference; lines connected to actual earth ground (that is, a metal rod driven into the earth) should be at exactly zero volts potential, always and undeniably. If a ground line reads some voltage above zero, that means that it has become disconnected or damaged somewhere between where you are reading and the outside ground connection. The ground line is included in household wiring basically as a safety feature: it gives stray energy somewhere safe to go.
'Neutral' should be equal or close in potential to ground. It is actually the 'center point' between two phases of electrical service coming to your house from the transformer on the electric pole; the lines from the pole are 240 vac apart, and in your circuit breaker box these supply two rows of breakers. One line supplies one row at 120 vac, the other line supplies the other row at 120 vac, with the neutral tapped betweeen them at (ideally) zero vac. The large loads in your home, like the air conditioner, that run on 240 vac power tap from both sides of the breaker box (straddle both buss bars). If you have ever heard an electrician comment on a home's electrical load being 'balanced', he is referring to whether the electrical current drawn from each 120vac line is equal or not. If they are unequal, then the 'center point' tends to shift away from zero volts, and thus so does the voltage on your 'neutral' lines.
'Hot' is exactly what it sounds like: it is the line with the 'juice' that supplies the energy to everything. It is supplied to the devices in your home through a set of circuit breakers (or fuses in much older homes). It is wired directly to the outlets, and goes to the permanently installed lighting through the permanent wall switches.
The reason you need to know this is simply this: if your home is mis-wired, you have the potential for personal injury or fires. Checking this is very simple. There is a little device sold at most building supply stores (and some department stores) which allows you to check if your outlets are properly wired. It usually looks like a small yellow or orange plastic 'wedge', with three lights on one end and a three-prong plug at the other, and is about the size of a salt shaker. These things cost a few dollars. When one is plugged into an outlet, the lights light up in a coded pattern that tells you if it is wired correctly, and if not, how not.
All new homes (in the U.S.) are supposed to be wired according to the most recent National Electrical Codes (NEC), and usually are. However, many older homes are not, the older homes being more likely to not meet the standards. These standards specify wire gauge sizes to safely carry given currents over given distances, color-coding for wiring, etc. The 'standard' colors in house wiring are: Hot = black; Neutral = white; ground = green, or bare wire. On outlets (and device plugs), the short prong is hot, the wide prong is neutral, and the round prong (if present) is ground.
You may have noticed that some devices (and tools and extension cords) have only two flat prongs, and some have the third, round grounding prong. The two-prong devices are typically low-current, low-power devices like lamps, Christmas lights, clock radios, can openers and the like. The third prong generally appears on higher-powered devices: washing machines, vacuum cleaners, televisions, computers, leaf blowers, drills, etc. This is usually done for safety reasons; let me explain.
Power tools and other hgih-power devices with which you make intimate contact often have metal surfaces or controls; even your 'double insulated' hand held power drill usually has a metal trigger. All these consumer devices are designed so that, if the internal wiring shorts, or the primary insulation fails, the exposed, consumer-accessible parts are still grounded, so that no wayward current can find its way to harm the user. This is why you should never defeat the installed ground prong on devices, or use a 'faker' adapter that allows you to plug a three-prong device into a two-prong outlet - it provides absolutely no personal protection in case of equipment failure. This is also why mis-wired outlets are dangerous: if the neutral and hot are reversed, the appliance will work just the same. But, if the appliance fails, the 'stray' energy is now under the 'top' insulating layer (next to the user), NOT the bottom one, putting the user at much greater risk. In older tools with metal cases, the user would actually get shocked in this event.