Blacksmithing is one of WoW's crafting professions - it requires materials produced with another profession, usually Mining. Blacksmiths turn various reagents (mostly metals) into plate armor, weapons, and some utility items.
Unlike some of the more "magical" professions in the game, blacksmithing functions in a more direct fashion. While professions like Enchanting or Inscription provide subtle buffs to your character, blacksmithing hands you the item you need. Right there, a new piece of gear. The one exception from the rule is - Elementium Rod, as well as the lower level enchanting rods. All Enchanters need to craft their own Runed Rods, a key ingredient for which is the blacksmith-crafted base rod.
Aside from uncommon, rare, and even epic armor and weapons, blacksmithing doesn't have much else to offer. To compensate for this, however, blacksmiths have access to some very solid pre-raiding gear, as well as new plans as the major content patches roll over. This is done to make sure that plate users will have need for blacksmiths as long as they need gear.
Profession Perk
Blacksmiths receive two additional sockets to their gear - Socket Bracer and Socket Gloves. The materials required are practically free, which makes the sockets also free.
Specializations
Blacksmiths no longer have specializations. If you are interested in the past of the profession, you could look into the 5 specializations smiths used to have: Armorsmithing, Weaponsmithing, Master Axesmithing, Master Hammersmithing, and Master Swordsmithing.