What are Flex "spark" layouts?

In a Flex application, we usually have multiple components which are arranged in some layout in our applications. They may be horizontally laid out, or vertically, or in a combination of horizontal and vertical layouts, with groups of vertically laid out elements interspersed between horizontally laid out elements. This is accomplished by having different containers which have different layouts, and by placing one container within another very complicated layouts can be accomplished.

In the new component architecture called "spark" which has been adopted in Flex 4, the layout mechanism of containers is kept separate. So while the Group container, TitleWindow container, List container all have different features, and looks they can be assigned any layout we want,
and the way they place components will be derived from that. So while the Group and Panel components are simply for displaying components, and the List component is for displaying and navigating through data, they can use the same layout. In our last example we used a TileLayout with a List to display our gallery.