Actually Guru64 roms and ISO's are essentially exactly the same thing.
If you want to get technical an ISO is actually a file type, which was originally used to make images of cd's. Of course since then it's been expanded to just mean a image of pretty much anything, be it a CD or DVD. Even though it is technically just one image format. Much like how people refer to zipping things instead of compression things when they use alternative formats like rar.
ROM on the other hand is defined as and I quote "ROM images (or ROMs, for short) is used in the context of emulation for a binary file which contains graphics, sounds, and program code. ROM images are usually created so that the software can be run on computer hardware different from the one the software was originally designed for, using emulator software. "
Which means as long as it's a binary file an ISO is just as much a rom as anything else.