An Apple Disk Image is a proprietary disk image format commonly used with the Mac OS X operating system. The format allows secure password protection as well as file compression and hence serves both security and file distribution functions; it is most commonly used to distribute software over the Internet, Apple Disk Images typically have a .dmg extension, although legacy Apple Disk Image files intended for Mac OS 9 and earlier generally have .smi or .img file extensions. When opened, they are "mounted" as a drive within the Finder.
Apple Disk Images can be created using utilities bundled with Mac OS X, specifically Disk Copy in Mac OS X v10.2 and earlier and Disk Utility in Mac OS X v10.3 and later. These utilities can also use Apple Disk Image files as images for burning CDs and DVDs. Disk Image files may also be managed via the command line using the hdiutil utility.
Apple Disk Image files are published with a MIME type of application/x-apple-diskimage. Many web server administrators may be less knowledgeable about Apple-specific file types, so this MIME type is often not set; this results in the user experience of attempting to download Disk Image files as text directly to the browser window, requiring explicit download commands to download the file. For this reason, Apple Disk Image files are often distributed inside bzip2 or ZIP files to ensure that the files are handled correctly by the server and browser software.