Installing Software
When you want to install a piece of software on the Mac, there are several ways to do it -- the way you use depends largely on how the software company packages their program. This is the most (for now) common method.
1. Download the software (in this example, Skype) from the company's website. When it's completed downloading, you'll have a file ending in ".dmg" (Disk iMaGe) in your Downloads folder or on your Desktop, wherever your web browser saves things. Think of a disk image as a sort of "wrapper" file for the real contents -- kind of like .zip compressed archives. The icon will have a picture of a hard drive, like this:
2. Double-click that file. This will "mount" the disk image (i.e., open it up so that the contents are accessible.) You'll see a brief dialog box like this while it is mounting the image:
3. When it's mounted, you may see an additional icon on your Desktop, like this:
4. A window should appear (if it doesn't, double-click that new icon). The window will contain the new software and, usually, an arrow pointing to an alias (shortcut) of your computer's Applications folder:
This means that you should click and drag the program (Skype.app) into your Applications folder to install it. The short video below shows this process, as well as finding the newly-installed program and adding it to your Dock for easy access later. Note that, if you are updating an existing piece of software (as was the case in this example -- I already had Skype, but wanted a newer version) then you will be prompted about whether you wish to replace the existing copy. Go ahead and do so.
When you're finished, drag the program icon AND the .dmg file to your Trash, you don't need them anymore.