Question

does sp. def. guard against the sp. att. of other types of pokémon or does it gaurd against (say a psychic att. against a psychic)?

if sp. def. only guards against att. of its own type does def. cover everything else?

Answer 1

The higher you pokemon's def. the less damage it will take from other Pokemon's physical attacks, the higher your sp.def is, the less damage it take from sp. attacks. That is all the defensive stats do.