Blacksmiths take bars of metal from miners and work them into finely crafted arms and armor. There are many unique weapons and other armaments that cannot be found on any of the vendors or monsters in the game, but can only be created by blacksmiths. Blacksmiths can create weapons, mail armor for Paladins or Warriors, and trade items. Blacksmiths can construct special sharpening and weight stones that can be used to temporarily improve weapon damage. They can also create several items that are used as ingredients for other Profession recipes, such as Enchanting. Blacksmiths can also create a few items that are needed for quests. They can put these items in the auction house to generate additional income.
If you plan on being a blacksmith, you should probably already be a miner. Miners collect ore and smelt ore into bars to create the majority of blacksmithing items. Blacksmith recipes often include jewels, which are found in a variety of places. Typically, most blacksmiths buy or are given rare jewels from other players. Some blacksmithing recipes require leather, which you will need to buy from a skinner.

Blacksmithing is great for Paladins or Warriors, who can use the skill to create their own equipment. It is also good for making money by selling weapons and armor to the classes that can use them.

Becoming a Blacksmith

First you need to find a blacksmithing trainer. Ask a guard for directions in a major city. Blacksmithing trainers are usually located near a forge or anvil. After you have learned basic blacksmithing, open up your abilities book (hit "p") and look for the "Blacksmithing" icon that has been added to your abilities. Now you need to go buy yourself a "blacksmithing hammer." These are usually sold by a "trade merchant" or other merchants near the blacksmithing trainer. You must have your blacksmithing hammer with you whenever you create an item. Some items don't require you to be near an anvil to create them.

Basic Ingredients
Blacksmithing usually needs one of these ingredients: Weak Flux, Strong Flux, Coal, Grinding Stones, or Elemental Flux. Sometimes recipes require gems, cloth, or leather.
Plans
You can purchase plans at the trainer. Additional plans can be found from monster drops, chests, quests, specialty vendors, or at the auction house. You can unlock recipes via increasing your reputations.

Sharpening/Weight Stones
Blacksmiths can create sharpening and weight stones that can be applied to weapons to temporarily increase their damage.

Steel Weapon Chain
You can create a weapon chain that can be attached to weapons to make it impossible for them to be disarmed. Some players want this upgrade to their weapons (only one enchantment is allowed).

Shield Spikes
Blacksmiths can attach a shield spike to a shield that will cause an enemy to take additional damage when attacking.

Mithril Spurs
Blacksmiths can create Mithril Spurs and attach them to boots to increase mount speeds. This can be a money making opportunity.

Dark Iron
Dark Iron deposits can be found in the Burning Steppes, Searing Gorge, Blackrock Depths, and Molten Core. To learn how to smelt Dark Iron you must complete a quest for Gloom'rel in the Summoners Tomb. You must smelt Dark Iron at the Black Forge inside the Blackrock Depths instance. Dark Iron items must be forged at the Black Anvil in the same instance.

Specialization

At high levels, players have the ability to specialize as an Armorsmith or Weaponsmith. Additionally, Weaponsmiths can specialize as Swordsmiths, Axesmiths, or Hammersmiths. Decide what is more important to you such as weapons vs. armor. You'll be able to create specialized items that are very powerful and likely valuable. Ask your guild what type of Blacksmith they need, if you have one. Once you have chosen your specialization, you cannot switch to another one, even if you unlearn Blacksmithing.

Link: http://www.wowhead.com/?spells=11.164