DMG stands for disk image. It is a format commonly used to distribute files and applications among Apple computers. A DMG file is like a virtual DVD or hard drive. DMG files can be “mounted” on your Mac to work with their contents. They can even be burned into an actual physical disk.
Fig. 1 – A disk image mounted as a virtual drive.
In order to understand the concept of a DMG disk image, think of a storage volume such as a CD, DVD, hard drive, or external hard drive. A DMG file is like one of these devices in that it serves as a means to encapsulate documents, images, software, and other files. The difference between a DMG file and those mentioned above is that with a DMG, there is no physical storage medium.
Once you are done working with the contents of a DMG file, you will want to remove or “unmount” it from your system. Do this by opening the Finder and clicking the eject icon shown next to the virtual drive’s icon. You can also do this by going to your desktop, clicking once on it’s icon, and pressing CMD+E.