As a Canadian, I use both "cookie" and "biscuit". A cookie is soft, and usually made with some kind of inclusion -- raisins, nuts, or cream filling. A biscuit is crisp and may have icing or sprinkles on it. So a Jaffa cake is a cookie and a Hobnob is a biscuit. Shortbread and gingersnaps are biscuits. Sandwich-type confections (e.g., the Oreo) are cookies -- the filling, you see. And those things with chocolate chips in them -- cookies. Soft.

It's easy. Well, easy for a Canadian. Your mileage may vary.