In Chapter 2, Installation: Getting Joomla Up and Running, you installed Joomla with the example site data. You'll only do that once, when you're new to Joomla and want to have a first look at its possibilities. In the previous chapters, you've explored the sample content. Now that you are starting to build your own site, you don't need all that content anymore. Unfortunately, there is no "Uninstall Sample Data" button. You can install Joomla again without sample data, but in this case we'll just clean up the sample site.
Admittedly, cleaning up Joomla may seem about as exciting as wiping clean a wall-to-wall classroom blackboard before the lesson starts. However, it's a good way of preparing yourself for your very first Joomla site. You'll find that stripping Joomla's example site will give you some useful insight into the way it's constructed. Bit by bit, it will reveal the different types of content that have been used to fill the empty CMS framework...