Yelp:Introduction to Panels

Panels, an integral part of the Gnome interface, are highly customizable interfaces which give users tools that are best seen in front of all windows. Below is a more detailed, and slightly more complex definition from the GNOME Project's website.

"The Panel is a generic term describing one particular control interface between the user and the desktop environment. A standard scenario is to have a bar at the bottom of the screen, with a menu from which users can launch applications (the GNOME Menu), a button bar with buttons representing more launch targets, as well as running apps."

Top Panel
The panel that is present at the top of your screen by default, referred to here as the top panel, gives you access to the main menu, a space for iconic links to programs, a system tray (similar to that in Windows), a clock, and a button to turn off your computer.

Space in the center of the top panel is left free for user customization. One may drag icons onto the panel to create links to their respective programs, or even add applets.