This is also sometimes called The Rede of the Wiccae. It was first published as we know it in Green Egg magazine in 1975 (the Ostara issue - vol. III, no. 69 - in an article called 'Wiccan-Pagan Potpourri') when it was submitted by Lady Gwen Thompson (1928-1986), who apparently had it given to her by her grandmother, Adriana Porter (1857-1946). The Rede was apparently known a few years before this, when it was received by members of the newly-formed Georgian Tradition in 1970. The 'eight words' couplet ('Eight words the Wiccan Rede fulfil, an it harm none, do what ye will') is first known in about 1964, in a speech by Doreen Valiente, but the first mention of 'do what thou wilt' and 'no harm to others' in connection with Wicca is from Gerald Gardner's The Old Laws in 1953. So, nobody really knows how old the Rede is, but it is now a central pillar of the Wiccan faith.
So what is the Wiccan Rede exactly? Well, "rede" is an Old English word that means "counsel" or "advice", and the Wiccan Rede amounts to a set of 26 rules. When I say rules, I don't mean Ten Commandments-type law - the rules in the Wiccan Rede are not "Thou shalt not [insert sin here]" set-in-stone-tablets laws that will send you to Hell if you break them (there are consequences for breaking the rules in the Rede, but eternal damnation isn't one of them). The Rede is a collection of guidelines that suggest to you what you should and shouldn't do, in both a practical and magickal context. Some things are common sense (i.e. #21 - "With the fool no season spend or be counted as his friend") and some are magickal in nature (i.e. #3 - "Cast the Circle thrice about to keep all evil spirits out"). The Rede is meant to give advice in relation to what you do - so it has the same purpose as the Ten Commandments, but gives advice instead of orders.
1. Bide the Wiccan Laws ye must
in perfect love and perfect trust.
2. Live and let live -
fairly take and fairly give.
3. Cast the Circle thrice about
to keep all evil spirits out.
4. To bind the spell every time,
let the spell be spake in rhyme.
5. Soft of eye and light of touch -
speak little, listen much.
6. Deosil go by the waxing Moon -
sing and dance the Wiccan rune.
7. Widdershins go when the Moon doth wane,
and the werewolf howls by the dread Wolfsbane.
8. When the Lady's Moon is new,
kiss the hand to her times two.
9. When the Moon rides at her peak,
then your heart's desire seek.
10. Heed the Northwind's mighty gale -
lock the door and drop the sail.
11. When the wind comes from the South,
love will kiss thee on the mouth.
12. When the wind blows from the East,
expect the new and set the feast.
13. When the West wind blows o'er thee,
departed spirits restless be.
14. Nine woods in the Cauldron go,
burn them quick and burn them slow.
15. Elder be ye Lady's tree,
burn it not or cursed ye'll be.
16. When the Wheel begins to turn -
let the Beltane fires burn.
17. When the Wheel has turned a Yule,
light the Log and let Pan rule.
18. Heed ye flower, bush and tree -
by the Lady blessed be.
19. Where the rippling waters go,
cast a stone and truth ye'll know.
20. When ye have need,
hearken not to others' greed.
21. With the fool no season spend
or be counted as his friend.
22. Merry meet and merry part -
bright the cheeks and warm the heart.
23. Mind the Threefold Law ye should -
three times bad and three times good.
24. When misfortune is enow,
wear the blue star on thy brow.
25. True in love ever be
unless thy lover's false to thee.
26. Eight words the Wiccan Rede fulfil,
an it harm none, do what ye will.
What all that means:
1. You must practice Wicca with an attitude of perfect love and trust - if you are not sure it is right for you, perhaps you need to rethink your path.
2. Respect both people and Nature - treat them as you would like to be treated.
3. When casting the magick circle, it is generally walked three times. Three is magickal number - it stands for the three aspects of the God and Goddess, it is the number of creativity and also refers to the Threefold Law.
4. Witches cast spells in rhyme to keep the conscious mind focused while the subconscious focuses on the desire and taps into the energy of the Universe.
5. Sit back and be quiet - wise people don't blab, they listen.
6. When the Moon is waxing, go deosil (anticlockwise in Australia) around the magick circle to bring good things towards you: i.e. cast the circle deosil under the waxing Moon.
7. When the Moon is waning, go widdershins (clockwise) around the circle to drive away negativity: i.e. cast the circle widdershins under the waning Moon. Wolfsbane is dreaded because it is poisonous.
8. When the Moon has just become visible again after the Dark of the Moon, salute and welcome the Lady by kissing your first two fingers.
9. When the Moon is full, go after your heart's desire. When the Goddess is at the peak of her powers, you can ask almost anything of her.
10 - 13. These couplets describe elements associated with the four directions (North, East, South, West).
14 - 15. There are nine species of magickal wood that are burned in the cauldron during ritual - apple, birch, fir, hawthorn, hazel, oak, rowan, vine and willow. The elder is the Goddess' tree and because of this it is never burned - it is always spared.
16 - 17. The turning of the wheel is the cycle of the year - Beltane is the greater sabbat that occurs in the spring, Yule is the lesser sabbat that occurs on the Winter Solstice.
18. Take care of nature - respect it as a sign of respect to the Goddess.
19. When you throw a stone into water, many ripples spread out from it. In the same way, your actions send out ripples that affect everyone around you.
20. Don't let money rule you - never take money as payment for magick.
21. People are judged by the company they keep. If you hang around with a fool you may be seen as one too.
22. Enjoy the company of true friends.
23. Whatever energy you send out - good or bad, magickal or mundane - comes back to you threefold.
24. When you are in trouble, visualise a blue pentagram on your forehead - it will protect you.
25. Always be loyal in love, unless your partner is not!
26. These eight words sum everything up - if it harms nobody, do whatever you like.