FileMaker has a nice overview of IWP here: IWP overview
For more information about all this, open "fm11_Instant_Web_Publish_en.pdf" installed inside the <Language>Extras folder where FileMaker Pro is installed. Also available here.
Instant Web Publishing is an amazing technology, but it is far from WYSIWYG (what you see is what you get) and it can be frustrating to work in. Take your time, and don't hesitate to get in touch if you have trouble building something in IWP.