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.
What Does "Polarized" Mean?
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"?
How to Identify a Polarized Plug
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.
In point of fact, 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.
But My Plug Does Not Appear to Be Polarized
Yes, 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.
But My Outlet Doesn't Seem to Be Polarized, Either
Yes, 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.
What About Polarization on Three-Prong Grounded Plugs?
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 "backwards" or to get the wiring backward when replacing an appliance plug. This is good reason to be careful about DIY wiring, leaving it to a professional unless you are very confident of your skills.