The most common confusion among the new Users(and some old one as well). While all 3 are “Extensions”, They are quite different from each other.
A Component is the full page you see on front end. A Component is the largest extension in Joomla. It may offer a completely new functionality/features. Most components have features both front-end(site) and back-end(administration). When Joomla page is requested, a component is called to render the body of the page. Component have a menu item and menu item can have multiple components.
To Access components,
Admin menu > “Components”
Modules are lightweight and flexible extensions used for page rendering. Unlike components, modules are mainly used to show content around the pages. There are different kind of modules like Login and Footer Module. Modules are assigned per menu item. One can show or hide modules for particular pages and/or users. Other times, Modules can be assigned to components.
To Access Modules,
Admin menu > “Extension manager” > “Module Manager”
A Plugin is more advanced extension and it is essentially an event handler. When any of the Joomla part executes, be it the core, a module or a component, an event can be triggered. When an event is triggered, plugins that are registered with the application to handle that event execute. For example, a plugin can be used to detect spam comment.
To Access Plugins,
Admin menu > “Extension manager” > “Plugin Manager”