There are two types of electrical signals , those being alternating current (AC), and direct current (DC).
With alternating current, the direction electricity flows throughout the circuit is constantly reversing. You may even say that it is alternating
direction. The rate of reversal is measured in Hertz, which is the number of reversals per second. So, when they say that the US power supply is 60 Hz, what they mean is that it is reversing 120 times per second (twice per cycle).
With Direct Current, electricity flows in one direction between power and ground. In this arrangement there is always a positive source of voltage and ground (0V) source of voltage. You can test this by reading a battery with a multimeter. For great instructions on how to do this, check out Ladyada's multimeter page
(you will want to measure voltage
Speaking of voltage, electricity is typically defined as having a voltage and a current rating. Voltage is obviously rated in Volts and current is rated in Amps. For instance, a brand new 9V battery would have a voltage of 9V and a current of around 500mA (500 milliamps).
Electricity can also be defined in terms of resistance and watts. We will talk a little bit about resistance in the next step, but I am not going to be going over Watts in depth. As you delve deeper into electronics you will encounter components with Watt ratings. It is important to never exceed the Wattage rating of a component, but fortunately that Wattage of your DC power supply can easily be calculated by multiplying the voltage and current of your power source.
If you want a better understanding of these different measurements, what they mean, and how they relate, check out this informative video
on Ohm's Law.
Most basic electronic circuits use DC electricity. As such, all further discussion of electricity will revolve around DC electricity.