Package files are generally distributed in one of three formats that are supported by Joomla! These can be distinguished by the file extension used and are briefly described below.
These files are packed using the proprietary ZIP file format. This is the most widely used compression format on Microsoft Windows systems, but it can also be used on Unix-style systems. For more information see ZIP (file format) on Wikipedia
These files are packed using the TAR (for "tape archive") format, then compressed using the open source Gnu Zip file format. This is the primary compression method used on Unix-style systems, but is also supported on other platforms. For more information see gzip on Wikipedia
These files are packed using the TAR (for "tape archive") format, then compressed using the open source bzip2 file format. This is an increasingly popular compression method on Unix-style systems, but is also supported on other platforms. For more information see bzip2 on Wikipedia
How you go about unpacking a package file depends largely on what platform you are running on. Listed below are some of the major platforms and some suggested programs that you can use to unpack package files. See also Comparison of file archivers on Wikipedia.
XP users can right-click the downloaded zip package and select 'Extract all' from the menu. This opens the 'Extraction Wizard' which, by default, creates a new folder for the extracted data when the user continually clicks the 'Next' button followed by 'Finish'.
For more control over the zip process, XP users can download and install 7zip, a widely-used open source compression utility.
Vista users can also right-click the downloaded zip package and select 'Extract All...' from the menu. This then opens a window prompting you to select a folder in which to extract the files. Once chosen, simply click the Extract button.
Apple OS X users can double-click the downloaded .zip, .tar.gz, or .tar.bz2 archive. This will automatically create a folder with the decompressed files. You can also use the "Unix-style systems" instructions.