This is Easy Tabs. It uses jQuery to turn your block layout into tabbed content.
The license for this block was donated by it creator, Sébastien. Thanks, Séb!
Well, you could easily build a smaller website with just Easy Tabs for navigation; keeping all of your content on just one page.
For larger sites this makes sometimes confusing pages easier to understand by grouping content and displaying it in an orderly fashion. The external CSS file is easy to get and override should you so desire. Sébastien has made the tabs rounded with CSS3 and provides fallback for clumsy browsers but you can change this easily as well as make the rollovers and text size and padding to your individual tastes.
$20 in the marketplace. Good to have in your "toolbox".