Under Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

, a package is a directory

Directory (file systems)

In computing, a folder, directory, catalog, or drawer, is a virtual container originally derived from an earlier Object-oriented programming concept by the same name within a digital file system, in which groups of computer files and other folders can be kept and organized.A typical file system may...

 that is normally treated by the Finder as a single file, with the components of the object that the package represents being stored in files in the directory tree under that directory. Control-clicking (or right-clicking) on the package in the Finder, and selecting "Show Package Contents", allows a user to open up the bundle and see the contents.

Bundles containing executable code are usually packages. In addition, some documents are also represented as packages:

• Rich Text Format
  Rich Text Format
  The Rich Text Format is a proprietary document file format with published specification developed by Microsoft Corporation since 1987 for Microsoft products and for cross-platform document interchange....
  
   documents with images, which carry the .rtfd extension;
• Dashboard widgets;
• partially downloaded files in Safari
  Safari (web browser)
  Safari is a web browser developed by Apple Inc. and included with the Mac OS X and iOS operating systems. First released as a public beta on January 7, 2003 on the company's Mac OS X operating system, it became Apple's default browser beginning with Mac OS X v10.3 "Panther". Safari is also the...
  
  , which carry the .download extension;
• project files in GarageBand
  GarageBand
  GarageBand is a software application for Mac OS X and iOS that allows users to create music or podcasts. It is developed by Apple Inc. as a part of the iLife software package on Mac OS X.-Audio recording:...
  
  , Keynote, Pages
  Pages
  Pages is a word processor and page layout application developed by Apple. It is part of the iWork productivity suite and runs on the Mac OS X & iOS operating systems. The first version of Pages was announced on January 11, 2005, and was released one month later. The most recent Macintosh version,...
  
  , Numbers
  Numbers (software)
  Numbers is a spreadsheet application developed by Apple Inc. as part of the iWork productivity suite alongside Keynote and Pages. Numbers 1.0 was announced on August 7, 2007 and thus it is the newest application in the iWork Suite. Numbers runs on Mac OS X v10.4 "Tiger" or newer...
  
  , iMovie HD
  IMovie
  iMovie is a proprietary video editing software application which allows Mac, iPod Touch 4th generation, iPhone 4, iPhone 4S, and iPad 2 users to edit their own home movies. It was originally released by Apple in 1999 as a Mac OS 8 application bundled with the first FireWire-enabled consumer Apple...
  
   and Xcode
  Xcode
  Xcode is a suite of tools, developed by Apple, for developing software for Mac OS X and iOS. Xcode 4.2, the latest major version, is available on the Mac App Store for free for Mac OS X 10.7 , and on the Apple Developer Connection website for free to registered developers Xcode is a suite of tools,...
  
  ;
• Installer
  Installer (Mac OS X)
  Installer is an application included in Mac OS X which extracts and installs files out of .pkg packages. It was created by NeXT, and is now maintained by Apple Inc...
  
   packages.

NOTE: The new versions of Keynote 4, Pages 3, and Numbers 2 included in iWork '09 now save a document bundle as a compressed flat file by default. To view the contents of an iWork '09 document bundle, change the extension to .zip and decompress the file.

The Uniform Type Identifier

Uniform Type Identifier

A Uniform Type Identifier is a text string used on software provided by Apple Inc. to uniquely identify a given class or type of item...

(UTI) for a package is com.apple.package.