Extemporaneous means impromptu, or off the cuff.
Advantage: A good speaker can roll along with little preparation and usually finish before his audience becomes bored. One of the best all-round extemporaneous speakers was Isaac Asimov who, on his way to the stage with the chancellor of a big US university, asked, "By the way, what did you want me to talk about?" And the chancellor said, "About 20 minutes."
Disadvantage: If you're not a natural speaker or well-versed in your subject, your speech is likely to be boring. You'd have been better off with time for preparation.