Can you name some properties of the functor category Funct(R-Mod,S-Mod), where R,S are associative rings with unit?

EDIT: I am sorry for the lack of precision of my (first in MO) question. I was thinking in a sort of Yoneda's Lemma for functors $F\colon R\mbox{-Mod}\to S\mbox{-Mod}$, maybe using tensorizing functors $h_U$ to describe $Nat(h_U,F)$.

whatproperties? Is it very like a whale? – Yemon Choi Jan 28 '12 at 1:30