Some of the third party extensions available for Joomla include entire platforms for blogging, such as EasyBlog, eCommerce, such as ACEShop and Hikashop, and full fledged photo galleries like Phoca Gallery.
There are some extensions that are installed with the base Joomla installation; however, most can be found on the Joomla Extensions Directory. Many of the extensions found on the JED are non-commercial and free to use and modify, but some are commercial and must be paid for.
There are literally thousands of extensions to sort through on the JED. At the time of this writing, the Joomla! Extensions Directory states that they are, "Serving 9743 extensions to the community."
{jb_purplebox}Joomla extensions are explained further in lesson 12 of our free Joomla tutorial course - The Basics. We also go into greater depth with extensions in both Beyond the Basics and Joomla Pro.{/jb_purplebox}