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 (Figure 5-2). 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 file, 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 (middle top in Figure 5-2).
Figure 5-2. A: This is the downloaded compressed file. Double-click to get… B: The .dmg (disk image) file. Double-click it to “mount” the disk image. C: And now, the disk image itself. Double-click to open the installer window. (Dismiss the warning that the software came from the Internet.) “Eject” it after ...