Main Menu: You can access almost all of the standard applications, commands, and configuration options from the Main Menu. To add a Main Menu to a panel, right-click on any vacant space on the panel, then choose Add to Panel->Main Menu. You can add as many Main Menu objects to your panels as you want.

Menu Bar: You can access almost all of the standard applications, commands, and configuration options from the menus in the Menu Bar. The Menu Bar contains the Applications menu and the Actions menu. To add a Menu Bar to a panel, right-click on any vacant space on the panel, then choose Add to Panel->Menu Bar. You can add as many Menu Bar objects to your panels as you want.

System menus: System menus contain the standard applications and tools that you can use in the GNOME Desktop. The Applications menu and Actions menu are system menus. To add a system menu to a panel, right-click on a launcher in the menu, then choose Entire menu->Add this as menu to panel.