File Extension PKG
File extension PKG is used by files which store installation packages. Installation packages contain all compressed files and metadata necessary to install specific software. Most of the PKG files that you can encounter were produced for Mac OS and OS X software. But PKG files are also used on Sony Playstation 3, Solaris, Symbian and some other operating systems. If you'd like to open PKG file you need to do it on an operating system for which this PKG file was designed. Typically it will be Mac OS or OS X.
Technically PKG file is an archive with executable files and metadata files which software uses to install itself on the computer. So if you'd like to open PKG file without installing its content you can use WinRAR, 7-Zip or any other compatible file extractor.
You cannot install software in PKG file on incompatible operating system. Windows OS won't even open PKG files.