An .iso file is an image file which bunches many system softwares or application software into a package with a size of 650MB or so, a size of CD ROM, so that you can burn it into an CD ROM. Late on you can use this CD for installation of your bunched system and application softwares. A good examples is Linux source files. In addition to those .rmp files in tar.gz format, all system software are bunched into .iso files in order for you to download from website. For more details, pls try to search .iso howto.