An ISO file is a digital copy of a disk image that has been extracted from physical media. Creating ISOs is a great way to archive physical discs, which are likely to degrade over time. You can mount ISO files or explore their contents as if you had the physical disc in your drive. This is increasingly important in a world where disc drives are less common than ever before.
Steam Replays are great. They provide stats about your gaming habits from the last year, showing off your dedication to having a good time. But they also show every game you’ve played to other members of your Steam family so if you want to stop that from happening next year, you have to act now.