Websites built with TYPO3 are usually constructed by a number of "content elements" which are put together in a certain order or nested structure.
Each content element has a type. The type of the element determines what output it generates. The most popular types of content elements are "Text", "Text w/Image", "Search", "Login", "Insert Plugin" etc.
By combining the various content elements you can create almost any page layout you wish in a very flexible way but still with a lot of control over the content.
On the other hand you also have to deal with all the possible variations of content elements when you design a website. In order to help you design the stylesheet for your website all the subpages to this page contains various content element types and within each type some variations.