Ohm's law states that the voltage drop between the ends of a conductor (or resistor) is directly proportional to the current flowing through it, provided the temperature doesn't change. This is commonly stated as V=IR, where V is the voltage drop, I is the current and R is the proportionality constant (or "resistance").
Ohm's Law is not truly a law. There is a popular misconception among many less knowledgeable engineers that Ohm's Law always holds true; however, most substances do not display the proportionality described. In such cases, the voltage and current have a more complex relationship, which is described as a transconductance curve.
Indeed, no real-world device obeys this "law" perfectly. However, Ohm's Law is an adequate approximation for the behavior of many objects, such as strips of metal.