Difference between Kd and Km re: biochem

Hey,

I know Km is the substrate affinity constant and that a lower Km means stronger affinity/binding between E and S, but I am confused about Kd.

As I understand it: A + B -> AB and Kd = A + B/ AB (basically the reverse constant). What I do not get is how a lower Kd corresponds to higher affinty?

THanks