The best way to find out is to design a web page and export as CSS then check out the files :-) I find the best way to explore Fireworks is to try things out. I will still answer your question thugh. It will export a html file and a stylesheet.
Read this article for why
In short Fireworks facilitate CSS conversion of layouts. For example if you have a header fill that fills your header area horizontally. Fireworks do this through CSS Export without you having to do this manually in the page.