What is a pkg file and how do I open a pkg file?

There are several software systems that utilize the ".PKG" extension. PKG is an OS X Installer file. A .PKG file is the file format is used by Apple Inc. on its Macintosh line of computers and on the iPhone. The contents of a PKG file can be installed using the Apple Installer application. Apple's Newton operating system also used files ending in .pkg for Newton applications and programs. Because of this, when seen from the Mac OS X Finder, Newton applications appear the same as Mac OS X Installer packages, however they do not share their file format.
Double-clicking on the .pkg file will install the files. If the user right-clicks (Control-click) and selects "Show Package Contents" they will be able to preview the files stored in the package. Apple Installer is the built-in utility for opening PKG files included with Mac OS X.
The PKG file extension is also used in the Solaris, or Sun operating system (OS) to denote software packages that may be installed, removed and tracked using the Unix commands pkgadd, pkgrm,and pkginfo. Solaris is currently maintained and developed by Oracle Corporation, which acquired it through its acquisition of Sun Microsystems, which originated the OS. During the 1990's, the .pkg file suffix was also used in BeOS. Be bought the company called StarCode and acquired their packaging tools.
The .pkg file extension is also used by Sony PlayStation 3 on downloadable content over PlayStation Network. All game packages are signed with the ECDSA signature. Usually game packages are signed with two signatures - one signature is for the header and the other for the entry table.
The PTC/CoCreate 3D Modeling application uses .pkg files to store model files. This .pkg file uses the zip file format and can be opened using a zip utility software such as WinZip.