The Wiccan Rede (pronounced "read") is the sole Wiccan equivalent of the Christian Ten Commandments. This guiding statement of Wiccan ethics simply states that the individual may do anything that they wish, provided that they do not harm either themselves or someone else in the process. Of course, this mystical law is subject to interpretation, and is intended to foment thought on the part of the individual Wiccan. What may be harmful to one person may not be harmful to another, and vice versa. Additionally, what may be harmful to a certain person in one specific situation may not be harmful to that same person in a different situation. Each individual person and situation must be evaluated on its own merits.
Some things are right or wrong depending upon the circumstances. Very few acts, if any, are wrong all of the time. For example, Wiccans generally frown upon killing, but there is a difference between cold-blooded murder and killing in self-defense (some Pagans will disagree with this). There is also an ethical difference between killing animals for food, and killing simply for sport. Stealing is generally considered wrong and harmful, but perhaps not if such a thing was done to save someone's life, rather than for purely selfish personal gain. Suicide is likewise considered wrong by most Wiccans, yet giving one's life altruistically to save another person's life would usually be permissible, and even heroic. It must be pointed out that ethics are relative to a given culture in different time periods, and none should be written in stone.
Numerous other examples abound. For example, sex for recreational pleasure, when it is fully consensual for both parties, is not considered morally wrong to Wiccans, but when it is used abusively with intent to walk over someone else's feelings, or to manipulate another in some manner, then it becomes harmful, and thus morally objectionable. "Harmful" here is defined as when one knowingly manipulates another, and "harm" can be either physical, mental/emotional, or even spiritual. Speaking on behalf of one's religion, as another example, isn't harmful, but fighting to enact laws that will curtail other people's rights to seek their own path would indeed be spiritually harmful to another. In fact, one may often find themselves having to choose between what is legal in a given country, or what one believes is morally right. Violating human laws isn't necessarily harmful or unethical; it is in my opinion correct to do the right thing over the legal thing (not that I'm endorsing the breaking of any laws, mind you, since ethical or not, doing so can land you in jail, or being hit with a significant fine!). Taking dangerous drugs for recreational purposes with reckless abandon is undeniably harmful to oneself, but may not be when used for medicinal purposes under the careful direction of a physician.
Unlike Christian political-inspired theology, Wiccan beliefs do not terrorize its adherents with threats of fiery damnation in a fiery afterlife of eternal punishment called "Hell" after death in recompense for "sins" committed while alive, all of which encompass strict dogmatic rules which are indeed considered written in stone. Rather, the Wiccan Rede takes into account the Law of Threefold Return (often called simply the Rule of Three), a binding metaphysical law that emphasizes the principle of karma. Karma is an ancient Eastern doctrine that has been described as a metaphysical system of points earned for or against a person depending upon one's actions during a physical incarnation, and has an actual scientific basis in the Law of Cause and Effect, described in quantum physics. The points earned for or against you via karma can be either the result of mundane actions or with spellwork, both of which affect the material plane adversely. This law basically states that if you deliberately cause harm to another, or inadvertently cause harm to another person as the result of needless neglect on your part, harmful actions will return to you at some later point in either this life or perhaps even in a future incarnation, and by building up karmic debt you may have to "pay" a person back whom you slighted in this lifetime by helping them in another. The Rule of Threefold Return states that the harm will come back with triple the force that you used on the person in question, rather than in a comparable amount. In other words, according to the Law of Threefold Return, if you harm someone through nasty spellwork, someone will harm you; if you attempt to manipulate someone against their free will (such as love spells to win a specific person's heart, or to force a former lover to return to you), then someone will manipulate you; if you use spellwork simply to pump up your ego at someone else's expense, something will come along to crush it, etc., at three times the severity which you caused. Fortunately, of course, the reverse is also true; acts of love and compassion, as well as spellwork to heal or help someone in need (with that person's permission, of course), will also come back to you threefold in this life, or sometimes in a future incarnation.
Karma shouldn't be thought of as written in stone either, or as ultra-strict. As human beings, we are imperfect, and are expected by our higher selves to error several times during the course of our lives. The Wiccan Rede simply encourages us to attempt to be the best and most worthwhile person that we can be, and to achieve the highest goals of selflessness. Deliberate attempts to do otherwise will only lead to our destruction, both spiritual and otherwise. The Wiccan Rede is an encouragement towards good ethics, and even though we respect the Rule of Three, we need not let fear of it control our lives. We simply accept that negative actions begat negative consequences for us. This adherence to benevolent acts is the most important thing that any religion can offer a practitioner, and a reminder that spellwork is something to be respected and never to be abused, just like everything else during the brief years of each of our corporeal existences.
Below is featured the poetic version of the Wiccan Rede:
Bide the Wiccan Law ye must,In perfect love, in perfect trust,
Eight words the Wiccan Rede fulfill:
An ye harm none, do as ye will.
And ever mind the Rule of Three:
What ye send out, comes back to thee.
Follow this with mind and heart,
And merry ye meet, and merry ye part.