what is a .dmg file?

A .dmg file contains a device image; these files are often used to compress Mac OS X software so it can be downloaded faster, and offer options not present in some other compression mechanisms, such as checksum verification.

You get an additional icon on the desktop because of the mechanism used to handle these images; after opening the dmg the first time, you can drag the application inside to your hard disk, and you can then skip opening the dmg the second or subsequent time you want to use the applications.

(14161)

A .dmg file is a Disk Image. Think of it as a self extracting archive that doesn't put any actual files on your hard drive that you need to clean up after you install whatever is in it. Although you can copy the contents out of an opened disk image to the hard drive. It will then act as if you are copying from one physical drive to another.

That's one way to think of a .dmg file. But you can also consider it simply another way to compress and save files in a single file image. Similar to .zip or .sit.

Edit: Although Niel's explanation is more precise. 🙂

Once you've copied the contents of the .dmg to your hard disk, it is safe to drag the mounted image to the Trash followed by the dmg itself; afterwards, empty the Trash.

(14163)

Once you drag the icon to your desktop or applications, the .dmg file can go into the trash.

Just drag it back out from the applications folder to the desktop or your dock, it will copy itself. Done!!

You might find it helpful to keep the .dmg file after you have installed the app. It will save you having to download it again if you need to reinstall it for any reason (esp. important with "paid for" downloaded software). I save mine to my backup drive along with a small text file with any serial numbers or "activation" information.

I usually just save my receipts with the serial number/key. If you need to download again, just go to the site, download it and re-enter your key.

If you drag it to the dock it will make a copy (alias) there. You should just leave it in your applications and fire it up from the dock. Do you not care for using the dock??

moe.
The String Cheese Incident
Allman Brothers
Umphrey's McGee
New Monsoon
Perpetual Groove
Widespread Panic
O.A.R.
Gov't Mule
...and of course The Grateful Dead!! Just to name a few...

Oh yeah... I do realize this is not a chat room but it would rude not to answer his/her question.