A layout gadget is a gadget whose sole purpose is to contain other gadgets. Layout gadgets position and resize children according to a certain policy, taking the preferred size of the children into account. Gadget hierarchies are constructed by building up nested layouts.
Layout basics

Common layout gadgets:
Pack layouts
Track layouts
Grid layouts
Frame layouts
Book layouts

Advanced topics:
Glass layers
Manual layouts
Incremental layouts
Implementing layout gadgets

See also
Border gadgets