A full tang Katana or Sword means a part of the blade that is left unsharpened and extends down almost the entire length of the Tsuka or handle, making the blade and the handle the same piece of metal.
It provides the sword with certain rigidity and makes the blade an excellent and lightweight piece in combat and training. Depending on the carbon concentration, a full tang blade may be made from various steel types. Japanese swords or Nihonto are great art pieces and are often full tang blades.