Welcome to Basenotes, Man from Shalimar!
That's a great question, and I'm hoping one of our resident experts will answer. I'm a relative newbie, and I think of powdery fragrances as developing a dry, talc-y scent, similar to the dusting powder many women (and some men) used to wear (these days not so much). It's very hard for me to describe, though I could give you plenty of examples of fragrances that I think of as powdery (Arpege and Fleurs de Rocaille being two that spring to mind). Hopefully someone wiser than moi will come up with an answer.....^_^