The difference in names between US and British food usually is due to German and Dutch immigration in the top half of the country and Spanish in the bottom half, with a few places French too. Cookies per se are softer and often have nuts, dried fruit, or candy mixed in, or icing decoration. The current most popular is definitely the tollhouse chocolate chip cookie. If they're hard they are stale. Biscuits are made from very hard dough and cooked in a mold. A Brit would definitely recognize an Oreo as a biscuit.