.iso vs .img files for DVD images?

There seem to be multiple standards for image files representing the contents of a DVD. For example, iDVD creates .img files if told to burn a file rather than a DVD directly. However, Linux install disks (for example) that are too big to fit on one CD (over 740 Mb or so) usually come in the form of .iso files meant to be converted to a physical DVD using dd, Toast, Disk Utility, or a comparable program. Disk Utility can also mount an .iso image on the desktop so that it looks like a drive, much as with .dmg files. So, several related questions: * What exactly differentiates a DVD-size .iso from a DVD .img? * Is there a simple conversion path from .img to .iso? * Is .img an Apple-specific file format? * If .img is Apple-specific, should I be worried about archiving DVD images that I want to keep permanently as .imgs (i.e., is it possible that future software won't be able to read them)? The last question speaks to the specific issue that brought this question to mind--I have a number of DVDs of home movies, painstakingly produced with iMovie and iDVD, that I would like to archive both as actual burnt video DVDs (i.e. for a DVD player) and as image files from which I can burn more copies if necessary. But is .img the best format to do this, or are those likely to become obsolete? Would .iso be better? I'm interested in the general question of .iso vs .img (vs any other relevant formats) too, as the title of the question indicates.