There is only the DMG file, which can be written to a hard drive, burned to a CD or DVD, or sent over the Internet.
The difference is that with a DMG, there is no physical storage medium. A DMG file is like one of these devices in that it serves as a means to encapsulate documents, images, software, and other files. In order to understand the concept of a DMG disk image, think of a storage volume such as a CD, DVD, hard drive, or external drive. They can be “mounted” on your Mac in order to work with their contents, or even burned to an actual physical disc.
A DMG file is like a virtual DVD or hard drive.
