From Joomla! Documentation

The term extension is generic and the following specific extension types are available:

  • Componente (since Joomla 1.0) – adds custom functions to your site that can be selected from menus
  • Idioma (since Joomla 1.0) – defines an additional language for your site
  • Biblioteca (since Joomla 2.5) – provides functions to be used by other extensions
  • Módulo (since Joomla 1.0) – shows nonessential data in a side box, possibly on multiple pages
  • Pacote (since Joomla 2.5) – bundles related extensions
  • Plug-in (since Joomla 1.5) – modifies content in articles or provides functions to extend other extensions
  • Modelo (since Joomla 1.0) – define the look, feel, and navigation capabilities of your site