All items in the Ribbon commands region are displayed within pages (tabs) and this is a handy way to group items according to your logic. There can be as many pages, as you like.

Besides regular pages, which are always visible, the Ribbon control also supports contextual pages. These are painted differently than regular pages, and are designed to provide context-dependant commands. For instance, in the video below, a contextual Selection page is made visible when text is selected.