An extension is a software package that extends your Joomla! installation in some way. A small selection of extensions is included with the default Joomla! installation but many more are available from the Joomla! Extensions Directory. The term extension is generic and the following specific extension types are available (listed alphabetically): Component (since ) – adds custom functions to your site that can be selected from menus Language (since ) – defines an additional language for your site Library (since ) – provides functions to be used by other extensions Module (since ) – shows nonessential data in a side box, possibly on multiple pages Package (since ) – bundles related extensions Plugin (since ) – modifies content in articles or provides functions to extend other extensions Template (since ) – define the look, feel, and navigation capabilities of your site For an overview of the most important extension types and the functionality they provide, see Extension types (general definitions) Reference: http://docs.joomla.org/Glossary
A component is a kind of Joomla! extension. Components are the main functional units of Joomla!; they can be seen as mini-applications. An easy analogy would be that Joomla! is the operating system and the components are desktop applications. They are usually displayed in the center of the main content area of a template (depending on the template). Most components have two main parts: an administrator part and a site part. The site part is what is used to render pages when being called during normal site operation. The administrator part provides an interface to configure and manage different aspects of the component and is accessible through the Joomla! administrator application. Joomla! comes with a number of core components, like the content management system, contact forms and Web Links. Reference: http://docs.joomla.org/Glossary
A split menu is where different levels of a single menu are displayed in two or more locations on a single web page. For example, a common requirement is for a menu of top-level items to appear at the top of the page. When one of the items is clicked the user is taken to a page where a secondary menu, say on the left of the page, shows second-level items within the scope of the top-level item. The menus appear in separate locations on the page, but are related because one shows only top-level items while the other shows second-level items. This idea can be extended to include menus for third-level items and beyond. This can be implemented in Joomla using a single multi-level menu then creating more than one menu module each referring to a different level. Reference: http://docs.joomla.org/Glossary
