button

In the object model this is RibbonButton.
Is a single clickable item which could have an image and/or caption
Can be added to a group, button group, drop-down list, gallery, menu or split button.

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui"> 
  <ribbon startFromScratch="true"> 
    <tabs> 
      <tab id="CustomTab" label="My Tab"> 
        <group id="Group1" label="MyGroup"> 
           <button id="button1" 
                   label="Large Button" 
                   imageMso="HappyFace" 
                   size="large" 
                   onAction="Button_OnAction"/>  
        </group> 
      </tab> 
    </tabs> 
  </ribbon> 
</customUI> 

Attributes (properties)

C# Events (design-time)

The Button control has a Click event which can be accessed if you use the [[Visual Studio Ribbon Designer]]

' C#
void Button_Click(object sender, RibbonControlEventArgs e) 
{ 
} 
this.Button.Click += new Microsoft.Office.Tools.Ribbon.RibbonControlEventHandler(this.Button_Click); 

Callbacks (run-time)

' VBA
Public Sub Button_OnAction( _ 
   ByRef control As Office.IRibbonControl) 

End Sub 

' C# Equivalent
void Button_OnAction( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

'VB.Net Equivalent
Public Sub Button_OnAction( _ 
   ByVal Control As Microsoft.Office.Core.IRibbonControl) 

End Sub 

' VBA
Public Sub Button_OnGetDescription( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef Description As Variant) 

   Description = "my dynamic description" 
End Sub 

' C# Equivalent
string Button_OnGetDescription( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

' VBA
Public Sub Button_OnGetEnabled( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef ReturnValue As Variant) 

   ReturnValue = False 
End Sub 

' C# Equivalent
bool Button_OnGetEnabled( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

' VBA
Public Sub Button_OnGetImage( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef Bitmap As Variant) 

End Sub 

' C# Equivalent
System.Drawing.Bitmap Button_OnGetImage( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
   return new System.Drawing.Bitmap(Properties.Resources.untitled); 
} 

' VBA
Public Sub Button_OnGetKeytip( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef Keytip As Variant) 

   Keytip = "my dynamic keytip" 
End Sub 

' C# Equivalent
string Button_OnGetKeytip( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

' VBA
Public Sub Button_OnGetLabel( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef Label As Variant) 

   Label = "my dynamic label" 
End Sub 

' C# Equivalent
string Button_OnGetLabel( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

' VBA
Public Sub Button_OnGetScreentip( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef Screentip As Variant) 

End Sub 

' C# Equivalent
string Button_OnGetScreentip( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

' VBA
Public Sub Button_OnGetShowImage( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef ShowImage As Variant) 

End Sub 

' C# Equivalent
bool Button_OnGetShowImage( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

' VBA
Public Sub Button_OnGetShowLabel( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef ShowLabel As Variant) 

End Sub 

' C# Equivalent
bool Button_OnGetShowLabel( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

' VBA
Public Sub Button_OnGetSize( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef Size As Variant) 

End Sub 

' C# Equivalent
Office.RibbonControlSize Button_OnGetSize( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

' VBA
Public Sub Button_OnGetSupertip( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef Supertip As Variant) 

End Sub 

' C# Equivalent
string Button_OnGetSupertip( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

' VBA
Public Sub Button_OnGetVisible( _ 
   ByRef control As Office.IRibbonControl, _ 
   ByRef Visible as Variant) 

End Sub 

' C# Equivalent
bool Button_OnGetVisible( 
   Microsoft.Office.Core.IRibbonControl control) 
{ 
} 

ID vs Tag

It is best practice to use the ID attribute to identify which macro to run when a button is pressed.
However some people use the Tag attribute to hard-code the name of the macro in the Tag attribute.

Using the ID
In the XML, the ID attribute is used to identify the macro.

<button id="ID_Button" label="MyButton" onAction="Ribbon_Callback" /> 

The callback function uses the ID to identify which macro to run.
Using this approach means you can easily change the name of the macro later on.

Sub Ribbon_Callback(button As IRibbonControl) 
   Select Case control.ID 
      Case "ID_Button":  Call MyMacro() 
   End Select 
End Sub 

Using the Tag
In the XML, the Tag attribute is used with the name of the subroutine.

<button id="ID_Button" label="MyButton" onAction="Ribbon_Callback" tag="MyMacro" /> 

The callback function calls Application.Run.
Using this approach means you have to update the XML every time you want to change the name of a macro.

Sub Ribbon_Callback(button As IRibbonControl) 
    Application.Run button.Tag 
End Sub