What is the use of a ControlTemplate ?

It allows you to specify the visual structure of a class. The main advantage is we can override the default ControlTemplate defined in the control and replace it with new ControlTemplates, to reconstruct the visual structure of the class.

The following code illustrates the ControlTemplate of a button.

[XAML]

      <Style TargetType='Button'>
        <!--Set to true to not get any properties from the themes.-->
        <Setter Property='OverridesDefaultStyle' Value='True'/>
        <Setter Property='Template'>
          <Setter.Value>
            <ControlTemplate TargetType='Button'>
              <Grid>
                <Ellipse Fill='{TemplateBinding Background}'/>
                <ContentPresenter HorizontalAlignment='Center'
                                  VerticalAlignment='Center'/>
              </Grid>
            </ControlTemplate>
          </Setter.Value>
        </Setter>
      </Style>