iWeb saves your file as an iWeb Domain file. You have a couple of publishing options depending on which version of iWeb you have.
iWeb '09 (3.0.4) you can publish to an FTP Server or a local folder. ( With the built in FTP in iWeb you will end up with an address like "www.YourDomain.com/sitename/Home.html )
iWeb '08 you can publish your website to a local folder
However you publish your web site you will need a web host in order to get your site up on the internet. I personally use IX Web Hosting but, there are many paid & free options out there to choose from.
"I may receive some form of compensation from my recommendation or link."