aA + bB -> cC + dD
Law of Mass Action tells you that [C]c[D]d/[A]ab = constant only if the the above reaction is at chemical equilibrium, ie.
aA + bB <-> cC + dD
The equilibrium law is a specific form of the law of mass action. The equilibrium law tells you that for a chemical system at equilibrium, its equilibirum constant can be computed as [C]c[D]d/[A]ab. Law of Mass Action confirms that this value is indeed a constant.