An iFrame is actually another piece of HTML code running in HTML code. To make it simpler, an iFrame is a web page running inside a web page while the SWF is the format for the Adobe Flash which represents the output file from a FLA source with all the code and graphics put together. A SWF contains Action Scripting, graphics, audio files, animations and more.
For example, with the proper code you can embed a SWF to be displayed in an iFrame on HTML page. Check Google for such examples, because there are millions out there.