A disk-based Web site is a site you create on your local hard drive. After you design and work on the site, you can later publish it to a remote location like a Web server. You can do this using FrontPage or any other kind of publishing software.
Disk-based sites are the most commonly used because they have a clear advantage: you can edit and test them on your hard drive before they go live. This is something that you will definitely want to do if your site has the potential to impact people in any kind of substantial way.
For example, it's probably wise to create and test a disk-based site if it lists product prices. You'd want to test and review the pricing pages to make sure each price is accurate. Another example would be if someone is paying for an advertisement on your site. You'd want to test the appearance and the hyperlink for the ad to make sure it goes to the right place, right?
Note Some FrontPage features rely on the server and cannot be tested when using a disk-based site. For example, if you insert a hit counter, it will not function on your local hard disk, but it will when you publish your site to a server.