Definition: The Ribbon is the strip of buttons across the top of the main window. Here on the Ribbon you will find access to anything the program has to offer. You no longer have to hunt endlessly through many menus and sub menus to find the command you want. Commands are located in logical places now, unlike in earlier versions of all things Microsoft.
Here is an example - If you want to do something about the design of your presentation, you will access the Design tab on the ribbon. Following that, there are sections across the ribbon pertaining to things to do with design -- such as the background. If you want to change the background, click on the background text entry and anything you want to do with the background will appear in a contextual menu.
Examples: Mary wanted to insert a picture into her PowerPoint 2007 presentation so she accessed the Insert ribbon.
More on PowerPoint 2007
