Extensions for Joomla 3 add specific functionality to the Joomla core installation. They are listed at http://extensions.joomla.org

Extensions may make complicated tasks much easier, expand on the basic Joomla text editor, improve website security, create shopping carts, assist with interacting with social web services, allow for easier interaction with web payment services, create interactive slideshows, or a number of other functions.

Joomla's core installation has the basic functionality to create a working website. It allows you to create articles, navigational menus, basic contact information, add modules for sidebars, and a number of other things that are required for simple functionality. The Joomla community relies on external sources to add any additional functionality to the site required for specific tasks. This functionality is not added to the core in order to keep in as lean as possible. Not every site needs the added weight and complexity that extensions can provide. 

The most popular Joomla Extensions are things like Akeeba Backup, JCE Text Editors, jHackGuard, VirtueMart, K2, jEvents and JomSocial.

When you research extensions for your site be sure to look at the reviews. The more popular extensions are those most likely to:

  • Keep up with Joomla updates
  • Have the best support
  • Work smoothly
  • Have the fewest security risks
  • Have a large community that can provide answers to your questions

Also look at the Joomla list for vulnerable extensions. This list shows extensions that have known bug issues or security risks. http://vel.joomla.org