Apple Disk Image
The Apple Disk Image format is a MacOS, OS X, and Apple II disk image format family (which may be stored in DSK (Apple II) or nibblized disk image format) preceded by a prefix giving format information, and often a trailer with comments and other metadata. The prefix begins with the bytes corresponding to the ASCII string "2IMG". The trailer usually has the ASCII string "koly" at the start of it.
Usually the extension .dmg is used for current OS X images, but .smi (Self-Mounting Image) and .img are also used (mostly for old MacOS disks), and .2mg was used for Apple II series disks (which can come in DOS 3.3 or ProDOS sector order).
Mac systems are able to mount these files as if they were separate disk units, and access their files through the Finder this way.