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 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.
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. You may have to ask other people where to find a trainer. 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.

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.

Information

There are hidden trainers throughout the world who can teach you new and special recipes.
You can also buy new recipes from other players or find them as drops from monsters.
There are special quests specifically for blacksmiths.
There are no recipes on the trainer past skill 250. You will need to do quests or find drops to get those recipes.
Blacksmiths can open locks by creating and using Skeleton Keys.
There are blacksmithing recipes available to seasoned blacksmiths via a series of turn-in quests.
There are also armorsmithing recipes that are available to master armorsmiths. All 3 recipes are gained via quest.
Additional Blacksmithing recipes can be found in encoded messages found on the members of the Twilight's Hammer in Silithus.