Home electrical circuits that feed electrical outlets are made up of 3 wires, Black (Hot wire), White (Neutral wire), and bare copper, (ground). A/C (alternating current) electricity does not flow in one direction only like DC power, it "alternates" between positive and negitive at a rate of 60 time per second also know as "hertz". The polarity of an A/C circuit is still very important, although there is no positive and negative as in DC, the wires have to be terminated correctly to make the circuit safe.

Electrical outlets are polarized, this means that they are made so you can only plug items into them in one direction, this is why one slot is longer than the other. The long slot is the neutral side (white), the short slot is the hot side (black), and the small round hole is the ground (bare).

You can check the wiring in your home using a digital multimeter.

Insert the digital multimeter probes into the long and short slots of the electrical outlet and read the voltage. You should read somewhere between 118 to 125 volts A/C.

Now move one probe to the ground hole and leave the other probe in the short slot (hot). You should get a reading that is the same as the first reading between the short and long slots. The Neutral and ground conductors are actually bonded together in the electrical system so they should be at the same potential. The difference between the neutral and ground is that the ground wire is not intended to carry any current, its purpose it to carry ground fault current in the case of a short or fault in a device.

Now measure between the long slot and the ground hole in the receptacle. The digital multimeter should show close to zero, it is normal to have a few tenths of a volt here but it should be very low and less than one volt. Because the ground and the neutral are at the same potential and are bonded together in the electrical system there should be no voltage measured between the two. If you get a reading of anything more than 1 or 2 volt you need your electrical system checked out, this can be  dangerous and should not be ignored.

You can buy electrical testers that just plug into the electrical outlets, these testers have lights on the front that tell you if the electrical outlet is wired correctly or not. These are ok but they don't give you the information that a digital multimeter will give you, plus a good meter is an invaluable tool to have if you are going to be doing electrical work.