In Joomla, a program that associates a path (URL) with internal Joomla queries. Normally a router contains two methods, build() and parse(). The build() method takes an array of commands and returns a JURI object. The parse() method reverses this and turns a JURI object into an array of commands.

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