Every religion, faith, or path has a base tenet that acts as a guideline for its worshiper's actions and behavior. Christianity as its Golden Rule, Hindu and Buddism have Karma, and Wicca has the Rede. It is written in many different forms but the most commonly used is, "An it harm none, Do what ye will." Basically a Wiccan/Wytch/Pagan can perform any act as long as the repercussions do not harm. Generally harm includes not only physical injury but emotional turmoil, manipulation, and any form of domination or control.
You can also find the Rede in mainstream philosophy, though it prefers to go by a different alias. The Harm Principle is a major tenet of the Utilitarian standpoint and was strongly backed by the English philosopher John Stuart Mills. He stated that "the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others." Going hand in hand with the Rede, the Harm Principle states that a person is free to do whatever he or she pleases, a right that not even government can impede, except for when their actions negatively impact another.
There is much debate over who is protected by the Rede. It is understood by all that it includes the individual who performed the action and other people. Those under question are animals, plants, and the planet in general (this one not as strongly since paganism also stresses eco-friendly lifestyles). The reason animals and plants are not automatically included is for reasons of sustenance. Therefore many believe that the hunting and butchering of animals and the gathering of plants for food is acceptable. Others choose to go vegetarian and vegan to avoid any harm on a living being. The cruel abuse and murder of animals for sport and the massive destruction of vegetation purely for personal gain however is not seen as acceptable by any pagan. The degree to which one interprets the Rede is extremely personal and varies on an individual basis.
The Rede is backed by the Threefold Law or the Law of Return: "Evermind the Rule of Three, three times your acts return to thee. This lesson well thou must learn, thou only gets what thou dost earn." The Law is often linked to Karma. The Basic idea is that every action as a repercussion, just like Newton's Law of Physics. The difference in the Threefold Law is that the consequences of an action, whether good or bad, tend to happen more like a three act play then a one hit KO. Simply put, "Everything happens in threes."
Both the Rede and the Threefold Law are mentioned in a poem called the Wiccan Credo. It is unsure exactly when the Credo was written or who wrote it, though there are some ideas. There are also many versions due to countless rewrites and conflicting interpretations. It is often refered to as the full version of the Wiccan Rede, which is a clerical error, but does have some merit since the Credo points out varies Do's and Don'ts of Wicca in a way similar to the Ten Commandments.
The Rede and the Threefold Law are one of the first things a novice learns when introduced to the Craft, due to its extremely personal and far reaching inplications. My suggestion to anyone new to paganism, whether you are starting a lifelong spiritual journey or you are simply an open minded individual looking for knowledge, is that you spend substantial time discovering your own personal meaning and interpretation of both the Rede and the Threefold law, because what you find will affect your actions from that point forward.