How can you have two html files on the same screen?

When Photoshop Elements creates a web gallery, it creates two files - index.html and ThubnailFrame.html. The later files sits at the bottom of index.html. When you click on a thumbnail image it changes the page under it. How does it work?

Is there a way of overlapping two html documents in Dreamweaver? A have a intermediate knowledge of Dreamweaver, so a simple explanation would be most helpful.

Create a Frameset page in DW (Plenty of info in Help) use this to split the screen into two sections. Load index.html in the top section (though I WOULD re-name it as it's best that the frameset page be called that) and ThubnailFrame.html. in the bottom section.