Files with the extension .dmg are Macintosh Operating System disk images, which means they are a copy of the contents in a storage device. These types of files can be opened on a Mac computer and are often used to install new applications or software.
Files that end in .dmg will contain information from an entire storage device, such as an external hard drive or flash drive. When a .dmg file is opened, the Mac computer will act as if a storage device is attached. Files with a .dmg extension typically contain packages to install software, but can also contain normal files similar to a .zip file.