The Rede is the Central Law of Wiccan religion. In short it states: "If it harms none, do what you want."  You may see it phrased in different ways. Some use the old English: "An it harm none, do as thou wilt". The Rede is seen by both Wiccans and outside observers as very similar to the Golden Rule, a belief that is found in nearly every religion. It should be noted that, while the Golden Rule forbids harm subjectively, the Wiccan Rede forbids harm absolutely. The concept of ethical reciprocity is not explicitly stated, but most Wiccans interpret the Rede to imply the Golden Rule in the belief that the spirit of the Rede is not just to do no harm, but to actively do good for one's fellow man as well as oneself. 

Different sects of Wiccans read "none" differently: some include the self, others include animals or plants, and so forth. The Rede also expressly rejects the concept of sin outside of harm to oneself or to another. The Rede is only a guideline which the individual must interpret to fit each particular situation. Partner to the Rede is the Rule of Three (also called the Threefold Law or or The Law of Return). It states that both the good and the evil that one creates in the world will be returned threefold (in joy or suffering). It is therefore seen as a pragmatic reason for ethical behavior and compliance with the Wiccan Rede. One of the better versions of it can be found in this short poem by Lady Gwen Thompson: Ever mind the Rule of Three Three times what thou givest returns to thee This lesson well, thou must learn Thee only gets what thou dost earn!

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The complete poem "Rede of the Wiccae" or "Counsel of the Wise Ones" by Adriana Porter is as follows:

Bide the Wiccan laws ye must, in perfect love and perfect trust.

Live and let live, fairly take and fairly give.

Cast the Circle thrice about to keep all evil spirits out.

To bind the spell every time, let the spell be spake in rhyme.

Soft of eye and light of touch speak ye little, listen much.

Deosil go by the waxing Moon, sing and dance the Wiccan Rune.

Widdershins go when the Moon doth wane, and the werewolf howls by the dread wolfsbane.

When the Lady's Moon is new, kiss the hand to Her times two.

When the Moon rides at Her peak, then your heart's desire seek.

Heed the North wind's mighty gale, lock the door and drop the sail.

When the wind comes from the South, love will kiss thee on the mouth.

When the wind blows from the East, expect the new and set the feast.

When the West wind blows o'er thee, departed spirits restless be.

Nine woods 'neath the Cauldron go, burn them quick and burn them slow.

Elder be ye Lady's tree burn it not or cursed ye'll be.

When the Wheel begins to turn, let the Beltane fires burn.

When the Wheel has turned a Yule, light the Log and let Pan rule.

Heed ye flower, bush and tree, by the Lady blessed be.

Where the rippling waters go, cast a stone and truth ye'll know.

When ye are in dire need, hearken not to others' greed.

With the fool no season spend, or be counted as his friend.

Merry meet and merry part bright the cheeks and warm the heart.

When misfortune is enow, wear the blue star on thy brow.

True in love ever be, unless thy lover's false to thee.

Eight words the Wiccan Rede fulfill, an ye harm none, do what ye will.

Source: Wikipedia - Wiccan Rede and Law of Three

Triple Moon Rule graphic Copyright © Robin Wood 1997, Used with Permission