A panel is a bar which can hold many items such as application launchers, windows lists, clock, notification area, applications menu...
The Xfce panel can manage any number of panels and allows you to customize them by adding/removing items, choosing their position and size, setting background images or transparency levels...
The Xfce Panel comes with a great number of plugins but you can also install a lot of third party plugins. most of them are available through the Xfce Goodies project.
