At the end of any appliance electrical cord, there is a plug with two or three prongs (blades) designed to fit into standard wall outlet receptacles. When there are three prongs, these are known as "grounded plugs," and they are designed to fit into wall outlets so the round bottom prong fits into the ground slot on the outlet (thereby connecting the appliance's wiring to the house's grounding wire system).
Heavier appliances, such as those operating heating devices or motor-driven devices, usually have a grounded plug with three prongs. Light-duty appliances, such as lamps, are unlikely to have grounded three-prong plugs.
Many plugs have only two prongs. On two-prong plugs, there is no ground wire connection, as it's deemed unnecessary. In these plugs, one of the blades of the plugs connects to the neutral wires running through the appliance, while the other blade connects to the "hot" wires. Similarly, your house's wiring system includes both hot and neutral wires. The system of hot and neutral wiring is what is meant when we speak of a house's electrical system as being "polarized." It simply means that there are both neutral and hot wires and that there is a directional flow to how the current runs through the system.
Safety suggests that, if possible, the neutral wiring on your appliance should connect to the neutral circuit wiring in the wall outlet, while the hot appliance wiring should connect to the hot circuit wiring in the outlet. How is this accomplished? What prevents you from plugging in appliances "upside down?"
Look carefully at the prongs on a cord plug, and then look carefully at the slots on an outlet. You should see that one vertical slot on the outlet (and one of the flat blades on the plug) is wider than the other. This is the neutral slot and the neutral plug blade. The configuration means that you can only insert the plug into the outlet slots in one direction—wide blade to wide slot, narrow blade to narrow slot. This ensures that the neutral wires in the appliance are connected to the neutral wires in the house circuitry and that the "hot" wires in the appliance and the hot wires in the house circuitry match up.
Most appliances would work if they were plugged in "upside down." But it provides a measure of safety for the appliance and house wiring to be aligned in terms of polarity. This is the reason why most appliance plugs are polarized—so they can only be plugged in neutral-to-neutral, hot-to-hot.
What Is Polarity?
When referring to the two prongs on a plug, polarity simply means they have different functions based on the directional flow of the current. The wider prong connects the neutral wires (wires carrying unused electricity back to the breaker panel), and the narrow prong connects the hot wires (wires carrying electricity from the panel to an outlet or a switch).
It is entirely likely that you have some appliances or other devices around your house in which the plugs have blades of equal width—nothing to distinguish neutral from hot. These are unpolarized plugs. Older lamps and other devices may well have unpolarized plugs, and some modern electronic devices, such as chargers on computers or cell phones, also have simple unpolarized plugs. It is generally not a problem to plug in an unpolarized plug to a polarized outlet in whatever way it will fit. But anytime you are replacing a plug on an older lamp or appliance, it's a good idea to install a new polarized plug.
This too is sometimes seen in older homes. When you find an outlet that has vertical slots exactly the same width, it means you are probably looking at wiring that is quite old. These unpolarized outlets can be very inconvenient since polarized plugs will not even fit into them. It's a good idea to have an electrician replace these outlets with proper polarized outlets.
Three-Prong Grounded Plugs
Have you ever seen an upside-down outlet? This is not a safety hazard. Often in commercial applictions and schools, the outlets are turned "upside-down" but are actually done per code. Upside-down outlets service a specific purpose, with the ground prong upward, so that if something falls off the wall and hits the plug inserted in the outlet, it exposes the ground and not the hot and neutral. However, a backward wired outlet can be dangerous. Devices with three-prong grounded plugs are automatically polarized since there is only one way to insert the plugs into a wall outlet. That is provided, of course, that both the wall outlet and the plug are wired correctly. It is not uncommon for DIYers to install wall outlets " backward" (hot on the neutral screw "silver" and neutral on the ho screw "brass or dark") or to get the wiring backward when replacing an appliance plug. This is a good reason to be careful about DIY wiring, leaving it to a professional unless you are very confident in your skills.