Fireworks comes with an excellent step-by-step tutorial that covers all the basics, so start with that. The files it creates are .PNG files, but for compatibility it exports webpages as a collection of slices, a bit like Illustrator's Save For Web functions. If you look at the source for that site, it contains lines like this:
<!-- fwtable fwsrc="splashpage2.png" fwbase="spldf" fwstyle="Generic" fwdocid = "742308039" fwnested="1" -->
fw stands for fireworks. It inserts lines like the above into the html code so that it knows how to refer to the master document on the production computer. It also make it easier for the whole Macromedia suite of web development tools to keep track of what each component is doing.