Apps can be installed on your Mac in several ways: you can find many of them in the App Store included in OS X, you can buy them in a store and install them from a DVD, and many apps are installed by downloading the app directly from the developer’s website. This last way of installing apps is very convenient, but can sometimes cause confusion, especially when dealing with the most common delivery format for this type of app, the .dmg.
A .dmg file is also referred to as a disk image. The simplest way to think of it is as a virtual disc, like a software version of a DVD or hard drive. If you were to walk into the developer’s offices to pick up the app, they would likely hand you a DVD with the app on it. A .dmg file is like a virtual disc that you can download directly to your computer.
The reason this is an good comparison to make is that, like a physical disc, a .dmg must be opened on your computer (the technical term is mounted), and then it must be ejected when you are finished.
For example, if I wished to install Google Chrome on my Mac, I would go to their website and click to download Chrome. The .dmg containing Chrome would be saved to my downloads folder. When I double click on the .dmg, it is mounted to my computer, and a finder window is opened.
There are two common ways to install an app from a .dmg. Many programs, including Chrome, only need to be dragged from the .dmg window to your Mac’s Applications folder. In this case, they’ve even included a shortcut to this folder, so I would simply drag Chrome onto the Applications folder shortcut in the same window. If a shortcut was not there, I could open my Applications folder by pressing Shift-⌘-A. Note that the app in the .dmg will not move from the .dmg, but will be copied into the Applications folder.
With other apps, you may see an installer file in the .dmg. If this is the case, just double click on the installer, then follow the installation instructions.
Once your app is installed, the .dmg needs to be ejected from your computer. You can do this by right clicking in the empty space in the .dmg window, and then choosing EJECT “(.DMG NAME)” from the drop down menu.
If you have already closed the .dmg window, open a Finder window. You will see the .dmg listed in the sidebar on the left under DEVICES, along with any other external drives or discs currently connected to your computer.
Click the eject symbol to the right of the .dmg to eject it.
Once the .dmg has been ejected, the original .dmg file that was saved to your downloads folder can be dragged to the trash.
Jason Merrill, Mac & iOS Consultant