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 4-2). And unless you’ve changed the settings, they arrive in the Downloads folder on your Dock.
Figure 4-2. 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 the installation is complete. D: Here’s the actual installer. After the installation, you can delete all this stuff (except maybe the .dmg file, if you think you might want to install the software again later).
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 4-2).
Disk images are 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 (top right in Figure 4-2). For example:
Double-click it to open it. The software you downloaded is inside.
Remove it from your desktop by dragging it to the Trash (whose icon turns into a big silver
key as you drag), highlighting it and pressing ⌘-E (the shortcut for File→Eject), clicking its 
button in the Sidebar, or right-clicking (two-finger clicking) it and then choosing Eject from the shortcut menu.You’ve still got the original .dmg file you downloaded, so you’re not really saying goodbye to the disk image forever.
When you’ve finished unzipping or unstuffing a downloaded file, you may have several icons on your desktop or in the Downloads folder. It’s generally safe to throw them away—the .sit, .tar, .gz, or .tgz file you originally downloaded, for example, or the .dmg icon.
The disk image icon, the one that contains the actual software or its installer (top right in Figure 4-2), is a phantom drive, held in memory, that will go away by itself when you log out. So after installing its software, feel free to drag it to the Trash (or highlight it and press ⌘-E to “eject” it).
Once you’ve got a disk icon on your desktop—either a pseudo-disk from a disk image or a CD or DVD you’ve inserted—you’re ready to install the software. You can install many OS X programs just by dragging their icons or folders to your hard drive (usually the Applications folder). Others offer a traditional installer program that requires you to double-click, read and accept a license agreement, and so on.
If you got a program from the App Store, it’s easy to delete from the Launchpad (Opening OS X Programs).
If you got it from some other source, well, there’s generally no Uninstall program. To uninstall a program, you just drag it (or its folder) to the Trash.
Some programs leave harmless scraps of themselves behind; to check for them, look for preference files or folders bearing the dearly departed program’s name in your Library folders (especially in Application Support) and in your Home→Library→Preferences folder.