Google says it's an "Abstract base class for panels that can contain multiple child widgets". It supplies some basic methods for panels with multiple cells such as getWidget(int index) and remove(Widget w). You need to know what you are doing if you extend this class. Currently, this is not the place to find the information you need.
| AbsolutePanel |
| CellPanel |
| DeckPanel |
| FlowPanel |
| HTMLPanel |
| StackPanel |