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.
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.