Package in OS X

Package in OS X is a file folder that may contain a hierarchy of files and folders, and is displayed to the user by the Finder as a single file. This is an abstraction that is unique to OS X compare with Windows and Linux. You can view package as an archive of files/folders into a single file, or a zip file without the compression.

Package is common in OS X. Some examples where it is used include OS X applications with .app extension; Keynote, Pages and Numbers documents; Rich Text Format documents with .rtfd extension created by TextEdit.

When you look at a package in Finder, it is presented as a single file. The purpose is to prevent user from changing the files/folders contained within. You can however read and edit the content of a package. To access the content of a package, right-click the package in Finder and select Show Package Contents from the contextual menu. 

Show package content in Finder

The Show Package Contents will appear only when the selected file is a package. It is normally not advisable to change the content of a package, especially for application packages. Do it only when you know what you’re doing.