Sometimes called 'pressure' or 'potential difference', voltage is measured between two points and is the force which drives current through a conductor.

Voltage is measured in volts, V. In some circumstances it is represented by the letter E

Current

Current is the quantity of electricity flowing in a circuit.

Current is measured in amps, A. In equations, current is usually given the letter I.

Resistance

Resistance is the proeprty of a substance which determines how easy or difficult it is for electricity to pass through it.

Resistance is measured in ohms, Ω. In equations, resistance is usually given the letter R.

Power

Power is the rate that electrical energy is transferred.

Power is measured in watts, W. In equations, power is usually given the letter P.

Relationship

Given any two of the values, the other two can be calculated.

For a fixed resistance, increasing the voltage will increase the current. Decreasing the voltage decreases the current.

For a fixed voltage, increasing the resistance will decrease the current. Decreasing the resistance increases the current.