Example of an iFrame where content from another page is viewed:

Javascript can be used to load a frame very easily. There are two ways to do this. Here's a link to an example demonstrating the load of a new page into the iFrame using Javascript (the code is also available on this page):
http://www.dyn-web.com/tutorials/iframes/load.php
Another really cool feature about iFrames is that they can be dynamically generated which makes them a great option for 'tooltips'. If you hover over text or an object a little popup window appears with a picture or text.
One more cool feature with iFrames is that you can write a script to change the content of the frame daily. This would be helpful if you want to show a tip of the day. The code can be found here: http://www.dynamicdrive.com/dynamicindex17/dowiframe.htm
As you can see, iFrames are another great way to help keep web page elements dynamic. I'm sure there are even more uses for iFrames that I haven't covered but this is a start.