Mowgli learns the Law of the Jungle from Baloo, who knows all of the laws and teaches them to Mowgli, at least in part because Mowgli is an eager student, whereas the wolves typically learn only the rules that apply the most to them. Laws are mentioned or defined occasionally, such as not killing anyone at a watering hole when a "water truce" is declared. Fear is implied to be the only "law" that applies equally to all animals.
In "The Law of the Jungle", a number of rules are laid out, although we are told that there are "hundreds and hundreds" more, plus these are only the rules that are followed by or pertain to wolves.
Most of the laws are guidelines for living, and how one is meant to interact with other wolves and other animals so as to minimize conflict. A few of the laws do provide a sort of overall authority; for example, the word of the Head Wolf is law in all judgments that are not immediately obvious, and the "haunch and the hump" of the law, i.e. its core, is to obey. Most emphatically, the law states that one must never kill man, i.e. a human.