While DMG files are usually downloaded from the Mac App Store, they can also be burned to a USB disk when you want to install a utility on another machine. DMG file is usually encrypted and compressed, unlike ISO, which is an uncompressed disk image format. When opened, it mounts a virtual disk on the computer, and is normally used for installing new applications. A DMG file is essentially Apple's format for mountable disk images in Mac OS X (macOS) computers.
