Extensions are the most visible aspect of any Joomla website. The word extension is commonly used to mean two different things with respect to Joomla. The most common usage is as an add-on software program that works with Joomla to add functionality to a website.
Everybody knows Joomla is a great CMS. But story does not end just here ! Joomla CMS also provide a great framework to built applications upon. You may have custom requirements e.g. Hotel Booking or Auction or Shopping cart, Joomla can handle all.