DMG is not a single file format, and has several sub formats. In Mac OS, a double click or using the command line of open(1) will similarly result in a mount. The DMG files are self-contained file-system images, and can be mounted by the hdiutil(1) command, which can also be used to create them (using its -create option). They contain a mechanism for loopback mounts. Apple Disk Image (commonly called d m g) is the de-facto disk image format on OS X and iOS.
