Extension:dmg
What is it? Macintosh OS X Disk Copy Disk Image File
Where’s it from? An Apple Mac OS X CD burner. A dmg file is a bit-for-bit image of CD-ROM, hard drive, or other disk full of data. A dmg file can be stored on your hard drive, mounted and used as if it was a drive itself by one of the programs listed above. A dmg file can also be burned to a CD or DVD disc, and the programs it contains can be run from disc. Dmg files come in several file system formats including HFS, HFS+, UFS, ProDOS, Linux, and Fat32 and so may also require special mounting software.
How do I use it? Mac OSX Disk Copy; Windows: dmg2img http://vu1tur.eu.org/tools/; Linux: DMG Automounter for Linux http://sourceforge.net/projects/dmgmount/
Can I safely delete or move it? Yes
Can it be dangerous? No
–David Hakala