Executable files in Mac OS X vs Windows

Extensions don't actually change what type of file something is, although they often do hint to the system what do do with a particular item.

DMGs are not executables, they are disc images. They are files that act as discs, which often contain the executables and other files for applications. When you double-click on one, the system mounts it and you then have access to the files inside.

Both Windows and OS X execute binary executable files, although they are in different formats. The big difference in what a user sees if using the GUI is that on Windows, you usually see the .exe executable file, which resides in a folder that contains other files and information that the program uses. On OS X, all those other files, along with the primary executable, are contained in a "bundle" (really a folder) with the extension .app. Double clicking on a .app bundle will execute the internal executable and load your application.

Outside of the GUI, in the actual file structure, Mac and Windows systems are really quite similar; these differences are mostly a question of how the system presents things to the user for manipulation in the GUI.

You will also be able to execute any file that has it's permissions set to executable, given that it actually contains a valid program of course.

.dmg files are the disk image files normally used to install Mac apps. Once installed, Mac apps usually have the extension of .app, but that extension is normally hidden from the user. Mac apps are packages which appear to the user as a single file. This makes it very convenient to move apps to a new folder or a new computer.