What is a .dmg file?
A .dmg file is a disk image, meaning it's an exact copy (raw block DATA) of a disk (CD, DVD, USB stick, etc...) or even just a folder wrapped in a disk image. DMG images have been the default file format for distributing software on the internet for Mac OS X users since the year 2000. DMG images are safe to store on any partition type, such as Windows (NTFS) and are safe to distribute over FTP, send by email, etc... as opposed to the previous default Mac OS disk image format DiskCopy which totally unsafe to copy over anything else than a HFS (Mac) partition. DiskCopy allowed for compression, which is still the case for DMG images but the latter also allows for encryption... which proves to be a double-sided sword when comes to universal support, especially under Mac OS 9. That is precisely why you should consider making CDR images instead to preserve Mac OS 9 and earlier disk images.
How to use or extract DMG images contents?
DMG images are mountable and burnable/clonable onto real media under any Mac OS X version. You can also browse and extract specific files from DMG images using small shareware utilities such as PowerISO. Mounting a .dmg file makes a new drive appear on your Mac OS X computer, effectively letting you use it as if you had the original media inserted (e.g. a CD-ROM).
DMG images can also be attached to some emulators but they are not universally supported so you might end up having to convert your DMG image to an ISO image in order to attach it to an emulator or to mount it under your OS (unless it's Mac OS X). Note that PowerISO (under Windows or Mac OS X) does that conversion for you in just 2 mouse clicks under a few seconds.
Under Classic Mac OS (before Mac OS X)
If the DMG image is compressed and/or encrypted, there is no way to mount it under Mac OS 9 and earlier.
Mac OS 8.5 to 9.2.2: For non compressed, non encrypted DMG images, try Toast 5 Titanium
Under Mac OS X
Simply double click the DMG image and it will mount on your desktop. Alternatively, use Disk Utility bundled with every Mac OS X environment: Launch Disk Utility > FILE menu > Open disk image...
Open it using PowerISO
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