The Chaos tool suite (ctools) is collection of useful methods intended to help Drupal developers create complex interfaces without duplicating too much effort.
Drupal 7 introduced a new Form API element: Vertical Tabs. When you set a parent element as this Form API type, then any child elements that are of type fieldset are displayed as vertical tabs instead of as in the traditional fieldset. Unfortunately for us developers, the documentation does not provide an example of how to use Vertical Tabs properly, but with some trial and error you can do it.
However, this new Vertical Tab Form API element doesn’t work in modal multistep wizard forms created with ctools.