The control panel is a docking window where child windows can be docked. There are global windows which display information about the current output image or the project, like the Histogram Window, and there are windows owned by the Active Node, like the Resize Attributes panel shown in the image.
The Histogram Window shows the histogram of the output image of the **Display Node
The Attributes Panel is the main user interface of the Active Node. This panel is updated when the active node changes.