The DMG file is the default image file used by the Macintosh operating system, replacing the IMG image file. Most commonly, the DMG file is used to deliver applications. The DMG is exclusively a Mac OS X file. According to FILExt.com, "on a Windows PC it's likely you have a Macintosh file and you won't be able to use what is in it." Extracting the file within the DMG is fairly simple, as the DMG mounts just like a real disk.
Double-click the DMG file. This will open the file with DiskImageMounter utility. A dialog window will appear, verifying the file and mounting it. Once mounted, the DMG will appear in the Finder sidebar under the "Devices" header along with the hard drive.
Open a Finder window (click the "Finder" icon in the dock). Highlight the mounted image file within Finder's sidebar. A list of the files within the DMG will appear in the main Finder window pane.
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Drag the files you wish to extract to the desired destination on your computer. If it's an application, drag the files to your "Applications" folder. Depending on the size of the files, it make take a minute to fully extract the files and copy them to your hard drive.
Unmount the DMG after you have extracted all desired files by clicking the small "Eject" icon next to the mounted image in Finder's sidebar. Additionally, you can right-click (Ctrl-click) the mounted DMG icon on the desktop and select "Eject."