Frames are pretty new and only supported by the latest browsers.
This is why you have to create a frames and no-frames version of your page if you choose to use them because only a few people can see frames. Frames let you split the screen into several different sections, each displays an html file. They can be really useful and make a site really easy to navigate around. However some people get a bit over excited and display so many different little frames that the page just ends up looking a real mess. I would be careful about how many frames you put on one screen.