World of Warcraft Blacksmithing gives you the ability to forge weapons, armour and other useful items out of metal and other reagents. There are many unique items that can only be crafted by blacksmiths which is the reason this WOW profession is so useful. One of the items they can create are sharpening stones which increases the damage for your weapon.
Because most of the items made by blacksmithing requires different metals, a recommend profession to compliment it is mining. The reason for this is because if you don't mine the items yourself they can be very expensive to buy.
The site below has some great WOW mining guides that you can give you a better understanding about this profession.
