Basic electronic theory.

Electrical potential.
Before electricity can flow, there has to be a potential difference.
Potential difference (PD) is a difference in electrical pressure between two points, and is measured in Volts. Potential difference (PD) may also be referred to as EMF (Electro Motive Force), but many people just refer to it as voltage. Being a difference in pressure, it does not go anywhere, it just sits there pushing.
In the circuit on the right, the battery supplies the potential (6 volts), and the switch is off. The voltage can be measured across the battery, but not across the bulb - so the bulb will not be lit.

Current.
If electrons move around a circuit, a current is said to be flowing. For a current to flow, the circuit must form a complete loop.
Although electrons flow from negative and return to positive, the convention is to assume that current flows from positive and returns to the negative.
Current is measured in Amperes, usually shortened to Amps.
In the circuit on the right, the battery supplies the electrical pressure (6 volts), and the switch is on. The circuit is complete, current (I) will flow, and the bulb will light. The voltage can be measured across the battery and the bulb.

AC/DC
An electrical supply may be either AC or DC.
AC (Alternating Current) refers to a supply where the current will change directions at regular intervals. The amount of times it changes from one direction to the other and back again every second is referred to as the frequency.
DC (Direct Current) refers to a supply where the current will always flow in the same direction.

Resistance.
Resistance is the opposition to current flow. Without resistance, maximum current would flow and something will either get damaged or even catch fire. Resistance is measured in Ohms.
In the circuit on the right, the bulb offers resistance. The current (I) flowing through the circuit will depend upon the resistance of this bulb.

Power.
Power is the amount of energy used, and is usually measured in Watts.

Relationships.
EMF (voltage), current, resistance and power are all related. If you have any two figures, the other two can be worked out.
  Potential (Volts) = Current (Amps) x Resistance (Ohms).
  Current (Amps) = Potential (Volts) / Resistance (Ohms).
  Resistance (Ohms) = Potential (Volts) / Current (Amps).
  Power (Watts) = Potential (Volts) x Current (Amps).