A battle-ready katana is made from carbon steel. It has been properly heat treated for correct hardness, and it has a full tang handle that provides added strength to the weakest part of the sword.
If a katana is made from stainless steel, it is for decorative purposes only. Stainless steel is brittle and not functional for battle use. Although stronger, carbon steel must be heat treated to achieve the right amount of hardness for functionality. The tang of a sword is the metal piece that extends down into and attaches the sword to the handle. A full tang is forged as part of the sword and sandwiches between two halves of the handle.