Google says it's an "Abstract base class for all panels which are widgets that can contain other widgets". It supplies some basic methods for panels which can contain one, or more than one widget. You really need to know what you are doing if you extend this class. Currently, this is not the place to find the information you need.
| ComplexPanel |
| HorizontalSplitPanel |
| HTMLTable |
| SimplePanel |
| VerticalSplitPanel |