What is Ohm's Law?

In short

Ohm's law says that the current through a conductor is proportional to the potential difference across it, if the temperature stays the same. It is written $V = IR$ .

In simple words

The potential difference $V$ is in volts, the current $I$ is in amperes and the resistance $R$ is in ohms. The law holds for metal wires and resistors at constant temperature, which are called ohmic conductors. It does not hold for a filament lamp as it heats up, or for a diode. The graph of $V$ against $I$ for an ohmic conductor is a straight line through the origin.

Example

A resistor of $20\ \Omega$ carries a current of $0.3\ \text{A}$ . The potential difference across it is $V = 0.3\ \text{A} \times 20\ \Omega = 6\ \text{V}$ .