So far in this tutorial, we have only used the built-in controls found in the WPF framework. They will get you a VERY long way, because they are so extremely flexible and can be styled and templated to do almost anything. However, at some point, you will likely benefit from creating your own controls. In other UI frameworks, this can be quite cumbersome, but WPF makes it pretty easy, offering you two ways of accomplishing this task: UserControls and Custom controls.
A WPF UserControl inherits the UserControl class and acts very much like a WPF Window: You have a XAML file and a Code-behind file. In the XAML file, you can add existing WPF controls to create the look you want and then combine it with code in the Code-behind file, to achieve the functionality you want. WPF will then allow you to embed this collection of functionality in one or several places in your application, allowing you to easily group and re-use functionality across your application(s).
A Custom control is more low-level than a UserControl. When you create a Custom control, you inherit from an existing class, based on how deep you need to go. In many cases, you can inherit the Control class, which other WPF controls inherits from (e.g. the TextBox), but if you need to go even deeper, you can inherit the FrameworkElement or even the UIElement. The deeper you go, the more control you get and the less functionality is inherited.
The look of the Custom control is usually controlled through styles in a theme file, while the look of the User control will follow the look of the rest of the application. That also highlights one of the major differences between a UserControl and a Custom control: The Custom control can be styled/templated, while a UserControl can't.
Creating re-usable controls in WPF is very easy, especially if you take the UserControl approach. In the next article, we'll look into just how easy it is to create a UserControl and then use it in your own application.