For those who still prefer the "ownership" feel of an ISO, GOG provides "Offline Installers." These are the modern equivalent of ISOs—single, executable files that contain the entire updated game and can be stored on a hard drive forever.
An ISO file is a "disc image" that contains a perfect copy of everything found on an optical disc. When you download a game in ISO format, you are essentially getting a digital clone of the original installation media. pc+iso+games+download+updated