Here is the list of main components of CMDUI.xml:
4. Contextual tab group
The next few sections briefly explain these components and their uses.
Tabs on the SharePoint 2010 Server Ribbon
Tabs: are the root of the Server ribbon. They contain one or more groups, and contain similar functions.
Groups on the SharePoint 2010 Server Ribbon
Every tab in the ribbon contains a series of one or more groups. Groups are used to associate controls with similar functionality. Each group is associated with a template that defines the layout of the group and how the group should appear based on the scale of the ribbon. The scale of the ribbon refers to situations where there are too many controls to show in the ribbon, for example, when the browser is not in a full-screen maximized state and is in a windowed state.
Group Templates on the SharePoint 2010 Server Ribbon
Group templates are used to define the different layout options for the controls within a group. Microsoft includes 29 group templates in the CMDUI.xml file (to locate them, search for the element at the end of this file).
Controls on the SharePoint 2010 Server Ribbon:
The ribbon would not be complete if users did not have anything to select or click. Controls are the items that live inside the ribbon that users can interact with. Controls reside within groups. These include things such as buttons, toggle buttons, check boxes, text boxes and many other controls. For a complete list of all the available controls, see Architecture of the Server Ribbon at http://msdn.microsoft.com/en-us/library/ee537017.aspx
Contextual Tab Groups on the SharePoint 2010 Server Ribbon:
Contextual tab groups are used to provide functions that are not global to the current context, such as the page. They appear only when certain circumstances have been met and contain one or more tabs.
Contextual tab groups hide functionality and menu choices from the user when they are not available, and appear when applicable. Other examples of contextual tab groups include the Editing Tools contextual tab group that appears when editing a wiki page, or the Picture Tools contextual tab group that appears when a picture is selected in edit mode.