Languages are perhaps the most basic and critical extension type. Languages are packaged as either a core language pack or an extension language pack. These packages consist of INI files which contain key/value pairs. These key/value pairs provide the translation of static text strings within Joomla! source code. This allows both the Joomla! core and third party components and modules to be internationalised. Core language packs also include an XML meta file describing the language and providing information about the fonts to use for PDF content generation.

Reference: http://docs.joomla.org/Glossary

An abbreviation for “Linux Apache MySQL Perl/PHP/Python.” These group of free and open source programs are used to run many dynamic websites. Most Joomla sites run with Apache and MySQL. All run with PHP. In addition, JavaScript is now an important part of the Joomla “stack.”