Installing software on a Mac is  different from installing software 
on Windows. There isn’t just one way to install applications on a Mac, 
either — there are several different ways, depending on the application 
you want to install.
New Mac users will likely find themselves asking a number of 
questions: Why isn’t every application on the Mac App Store? What is a 
DMG file, and why do I have to drag-and-drop the app’s icon after 
opening it?