First of All, What Is Joomla! Anyway?

Joomla! is a collection of php scripts, which together form a web application. This application is a content management system (CMS). A CMS is an application designed to create, edit, manage, search and publish various kinds of digital media - from text to mp3s. 

You can use Joomla! for a blog site, for an ecommerce site, for an informational non-profit organization site, for an entertainment site, for a social networking site, for a portal, for just about anything.

When you browse to a page on a Joomla! website, the application is invoked, it checks with the database to see what content goes on the page, and it calls on the template for the requested page to draw everything in the browser.

An important thing about Joomla! to realize is that all of Joomla!'s pages are dynamically created. Your pages are not created until a user browses to one of them, and then they are generated right then by the Joomla! application. Unlike a static HTML website, the information on the page comes from many different sources as the page is being drawn. The functionality of components and modules is in one location (/components or /modules) while the code to build them on the page properly is usually in another location (/templates/your_template/html/override_directory, and finally the code to style them properly is in another location (/templates/your_template/css/various.css).

A module is a small application usually displayed in one of a template's module locations (e.g. left column). Modules are found in /modules and are managed via the module manager (/administrator Extensions->Module Manager).

A component is an application function that puts information on the page - an article, a form, the registration page, etc. Components are found in /administrator/components and /components and are managed via the component area (/administrator Components).  The information you are reading right now is displayed in the component location on this page and was generated by the com_content component.

A template is repsonsible for drawing the joomla page.

A plugin is a very small program that extends the functionality of a module, component or plugin. They are found in /plugins and can be managed in /administrator Extensions->Plugins