What is the icon on the desktop after I install an application? I am very very new to Mac. Here is my question:

I have downloaded VLC player and then I clicked on the dmg file. I moved the VLC icon to Applications folder in the opening window.

Now when I start Spotlight and type VLC I can open the app which is great. But there is also an icon on the desktop. What is this? 

It looks like a hard drive icon but I am not sure. When I right click on it I see: 

Kind: Volume
Disk image: /Users/kt/Downloads/vlc-..dmg
Format: Mac OS Extended
Capacity: 134 MB
Available: 13 MB

Anyway what is this file? 
Is it like a Windows shortcut? 
Is it the file I downloaded in the first place? (But it was downloaded do "Downloads" folder)
This is the downloaded disk image. Many applications downloaded from the internet will come as an internet ready disk image, which means that it mounts and opens when the downloaded file is opened.

A disk image is a mountable image of any volume. Disk images can be images of physical volumes or virtual disks. Like a zip, dmg disk images can be compressed (and usuall are, especially internet ready disk images such as the disk images used to install software from the internet) or can be read/write like an actual disk. They are mounted in the same mount point as physical disks, but are virtual. When unmounting, the disk is ejected and the mount point deleted like a physical disk. As the disk image is a disk, it will be shown on the desktop if you have the relevant option selected in Finder preferences:



Copy the app to your /Applications folder (or wherever else you desire) then eject the disk image by right-clicking and selecting Eject (or through Finder). The mounted disk image is the .dmg that you downloaded, therefore can be deleted by deleting this disk image after unmounting.