Files versus Folders in OS X El Capitan
A file in OS X EL Capitan is what’s connected to any icon except a folder or disk icon. A file can be a document, an application, an alias of a file or an application, a dictionary, a font, or any other icon that isn‘t a folder or disk. The main distinction is that you can’t put something in most file icons.
The exceptions are icons that represent OS X packages. A package is an icon that acts like a file but isn’t. Examples of icons that are really packages include many software installers and applications, as well as documents saved by some programs (such as Keynote, GarageBand, Pages, or TextEdit files saved in its .rtfd format).
When you open an icon that represents a package in the usual way (double-click, choose File→Open, press Command+O, and so on), the program or document opens. If you want to see the contents of an icon that represents a package, right-click or Control-click the icon and choose Show Package Contents from the contextual menu. If you see an item by that name, you know that the icon is a package; if you don’t see Show Package Contents on the contextual menu, the icon represents a file, not a package.
Folders work like manila folders in the real world. Their icons look like folders, like the one shown; they can contain files or other folders, called subfolders. You can put any icon — any file or folder — inside a folder.
Here’s an exception: If you try to put a disk icon in a folder, all you get is an alias to the disk unless you hold down the Option key. Remember that you can’t put a disk icon in a folder that exists on the disk itself. In other words, you can copy a disk icon only to a different disk; you can never copy a disk icon to a folder that resides on that disk.
File icons can look like practically anything. If the icon doesn’t look like a folder, package, or one of the numerous disk icons, you can be pretty sure that it’s a file.