As Bro. Jim noted, the main (but not only) meaning of extemporaneous is "performed without preparation", or what some people might call impromptu (and is not limited to sermons). But probably more people (in relation to preaching) usually have in mind a sermon delivered without notes or manuscript. Very few sermons can truly be said to be delivered without preparation, though some are. I have delivered messages totally "impromptu", with no forethought on that particular sermon. For example, following up on the discussion of the hanging of Saul's sons right at the end of a Bible study, I rose and preached a message on the text (though I wouldn't recommend everyone try it at home

). It was not nonsense nor laziness, yet
very impromptu. But 30 years or so of Bible study behind it can hardly be called "without preparation", though there was none specifically for that message. And though I always preach without notes, such an incident as above has been the exception rather than the rule.
I'd like to add a few more cents. First, I would suggest that it may just be partly
your experience and not something fixed that "prepared" sermons are better than "extemporaneous" ones. I'd heard lots and lots of nonsense from both, as well as good ones from both. The truth is not bound in a method of delivery. Second, I would recommend that you who prepare sermons in minute detail be careful that you not get caught up in only "studying sermons" rather than studying the Bible. While I won't say my experience is right for everyone, over the years I have come to the pleasant experience of "developing my sermons" from my Bible study, rather than studying the Bible to develop sermons. Lastly, I think the thread starter wasn't really looking for whether we preach with or without notes.
I would say most of my "sermon types" would have to be textual - points developed based on a specific text under consideration. But at the same time, there is no reason sermons can't be topical, historical, or a number of other things. We have in our area a number of preachers who have developed the idea that sermons MUST be expository. Nonsense! I don't think there are any New Testament sermons that are clearly what we call expository today. And the idea that you MUST have a text has caused some confusion, where you have a lot of gents hunting a text for a "jumping off place", never to return to it or mention it again. But if one is preaching on the topic of baptism, for example, just take the topic and study it. There doesn't have to be ONE "text".
To continue on with John's question:
I'm not pastoring a church now, but when I was preaching every Sunday twice a day, I used Sunday nights as a study through a book of the Bible. So all of these sermons were known ahead of time. But not so much so that it couldn't ever be varied, or that a visiting preacher couldn't be invited to speak. Most morning sermons were textual as well, but not book-by-book studies or series.
ONE LAST GRIPE - something that I guess may bother me more than anything other than someone preaching falsehood is the overuse of alliteration. A few use it effectively; most don't. It can make my ears shut down to hear someone drone on explaining the meaning of the word they had to wrest to get it to start with the right letter, and explaining why they chose to use it. Just preach!
Now that feels better.
