A dependency property is a special type of property which can be set inside a XAML file. Common .Net properties cannot be used in XAML.

This is an example of a dependency property along with a common .Net property wrapper which usually is used to access the dependency property inside your .NET class object.

public Orientation Orientation
 {
     get { return (Orientation)GetValue(OrientationProperty); }
     set { SetValue(OrientationProperty, value); }
 }
 public static readonly DependencyProperty OrientationProperty = DependencyProperty.Register(
     "Orientation", typeof(Orientation), typeof(AnimatedItemsScrollViewer), new PropertyMetadata(Orientation.Horizontal));

Features of dependency properties:

can be bound to a view model via data binding
can be set in xaml
can be set via a style
can be animated (Hint: animation can only be applied to dependency properties, common properties cannot be animated!)

Hint: Dependency properties can only be defined in classes which derive from DependencyObject (directly or indirectly). All WPF UI Elements derive from DependencyObject.

Dependency Property