core-component-listComponents are units of functionality for Joomla. Components perform a specific set of tasks. . Certain components come with the initial core installation of Joomla, but when you upload an extension, chances are it will add an additional Component to your Joomla installation.  Joomla's documentation calls components "mini-applications".  

The tasks that a component performs are controlled by the settings on the Administrative Interface.  Most components will have a Front End aspect to them as well although some just add Back End features.

The core Joomla Components in the drop down list are Banners, Contacts, Joomla Update, Messaging, Newsfeeds, Post-installation Messages, Redirect, Search, Smart Search and Weblinks.  The Components are there if you choose to use them, but you may find that you use some and not others.  Some components will not be accessible until you Enable them with the Plug-In manager under the Extensions pulldown menu.

Each of these core components will be covered in the  joom3 Administrative Interface section.

If you have installed 3rd Party Extensions, you will likely see additional items in this list.

Learn By Example

Back to Susie Catlover and her web site All About Cats.  She will be using the Components section to set up Banner Ads on her site, to set up a Contact Form for people to fill out on her site to contact her with comments and questions. If she changes the name or location of pages on her website, she can use the Redirect component to redirect the URL of a previous web page to a new web page. That way, visitors won't get frustrating 404 Error pages.  It is also good for SEO. 

The Newsfeed Component allows you to easily add news feeds from other websites to your own site.  The Messaging Component allows you to set up private messaging between users.  

As her website grows, Susie will utilize the Smart Search Component so that people can quickly find content on a specified topic. Once Susie's website becomes popular, she will be able to analyse for what people are looking with the Search component. She will also be able to create, remove and monitor links to outside websites with the Weblinks components.

If Susie decides to extend the functionality of her website with Joomla Extensions, there will be more items added to the dropdown menu under Components.