Its the defining ability of an enhancement Shaman. But what exactly is it? How does it work? There certainly can be a lot of misconceptions about it.
Windfury Weapon Rank 5
190 Mana
Instant cast
Requires Melee Weapon
Imbue the Shaman's weapon with wind. Each hit has a 20% chance of dealing additional damage equal to two extra attacks with 445 extra attack power. Lasts 30 minutes
This is the imbue Shamans cast on their own weapons. 99 times out of 100 you will want to use this over Rockbiter, Flame Tongue, or Frostbrand.
There is also a Windfury Totem.
Windfury Totem Rank 5
325 Mana
Instant cast
Tools: Air Totem
Summons a Windfury Totem with 5 health at the feet of the caster. The totem enchants all party members main-hand weapons with wind, if they are within 20 yards. Each hit has a 20% chance of granting the attacker 1 extra attack with 445 extra attack power. Lasts 2 min.
A shaman should never use this to imbue his own weapons as it only grants a single extra attack. Still this totem is so good that every DPS warrior, Rogue, and Ret Pally in the game will beg to be in your group in a raid. It flat out increases their white dps by around 20%.
One thing every shaman needs to be aware of is that the weapon imbue version of Windfury has a hidden 3 second cooldown. So using almost any 1H weapon in the game a shaman can never have consecutive WF procs. This also results in slightly under 20%proc rate for 1H weapons.
The implication of this cooldown is very straightforward: it is in the best interests of the Shaman to wield weapons whose speed is as slow as possible while still dual wielding. The reasoning for this is also straightforward - the white damage done by a weapon of any speed that has the same DPS as any other weapon of a different speed with the same DPS, so you cannot modify your white damage contribution from your weapon. You must maximize your extra attacks - in this case, maximizing the damage that each Windfury proc and Stormstrike attack will cause. Since neither WF procs nor Stormstrike are normalized (using a flat AP value regardless of weapon speed for a given class of weapon) they both hit much harder when using slow weapons. Under a perfect set of circumstance the maximum proc rate for Windfury Weapon is 20ppm, though it is typically around 15 ppm when wielding slow weapons -- this is far from a trivial contribution to your damage.
Also note that the chance to proc for WF when dual wielding is 36% as long as both weapons have the windfury weapon imbue.
The Windfury totem does not have a 3 second cool down. In no circumstance believe that the lack of a cooldown can make up for the difference in getting that 2nd extra attack. Never rely on the totem for your own weapons.
An enhancement shaman also has talents that improve both the weapon imbue and the the totem.
Elemental Weapons Rank 3
Increases the damage caused by your Rockbiter Weapon by 20%, your Windfury Weapon effect by 40% and increases the damage caused by your Flametongue Weapon and Frostbrand Weapon by 15%.
Improved Weapon Totems Rank 2
Increases the melee attack power bonus of your Windfury Totem by 30% and increases the damage caused by your Flametongue Totem by 12%.
So now your weapon imbue gives you two extra attacks with a bonus 623 AP and the totem gives a bonus attack with 578 extra AP.
And that folks is why everyone loves Windfury. :)
Thousands of dead animals later I had enough leather, hides, and scales stockpiled to drop skinning and powerlevel Leatherworking. Cost to me was about 350 gold. Almost all of that was spent on training and patterns. I spent maybe 25 gold at the AH to fill in a a few items like buckles and elixirs, and silk bolts used in making a few items. 
