In general, new programs arrive on your Mac via one of two avenues: as an Internet download (whether from the App Store or not), or on a CD or DVD.
Programs you download from the Web (not the App Store) generally arrive in a specially encoded, compressed form. And unless you’ve changed the settings, they arrive in the Downloads folder on your Dock.
The downloaded file’s name usually has a file name extension like .zip (the standard compression file format for Windows and Mac files) or .dmg ( a disk image, described below).
You may occasionally run into .tar files (tape archive, an ancient Unix utility), .gz (gzip, a standard Unix compression format), or combo formats like .tar.gz or .tgz.
Fortunately, you generally don’t have to worry about any of this; most Web browsers, including Safari, automatically unzip and unstuff downloads of all types.
Once you’ve downloaded a program, it often takes the form of a disk image file, whose name ends with the letters .dmg (second from the top in Figure 5-2).
Disk images are extremely common in OS X. All you have to do is double-click the .dmg icon. After a moment, it magically turns into a disk icon on your desktop, which you can work with just as though it were a real disk (third from the top in Figure 5-2). For example:
Double-click it to open it. The software you downloaded is inside.
Figure 5-2. Downloading a new program may strew your desktop or Downloads folder ...