What is the difference between oxidation and reduction in simple terms?

Think of it in terms of electrons, and the phrase OIL RIG helps: Oxidation Is Loss, Reduction Is Gain (of electrons). When a substance loses electrons it is oxidised; when it gains electrons it is reduced. The older idea also works: oxidation is gain of oxygen or loss of hydrogen, while reduction is loss of oxygen or gain of hydrogen. For example, when magnesium burns, Mg loses electrons to become Mg2+, so magnesium is oxidised, while oxygen gains those electrons, so oxygen is reduced. Both always happen together in a redox reaction, you cannot have one without the other, because the electrons lost by one substance are exactly the electrons gained by the other. Identify what loses and what gains electrons and you have your answer.