Joomla is one of several open source Content Management Systems. Drupal and Wordpress are a couple of others. The HTML for the webpages that you see is not generated by me directly, but by the Joomla system, by combining information in a database (content) with formatting from a template. The arrangement and colour of the text and menus is controlled by the template and is completely independent from the content. I can change the look of the website by changing the template without having to re-write any content. This separation of data and format is something that warms the heart of most programmers!
There are more extensions and templates than you can possibly imagine. If you want to know more, have a look at www.joomla.org and http://en.wikipedia.org/wiki/Joomla