What is and where need DependencyObject and dependency property

1.a Dependency Property is like any other property but can hold a default value,
  with built in mechanism for property value validation and automatic notification for changes in property value .
  any binding in silverlight is to binded to a Dependency Property.
  Dependency properties can hold an optional default value, and can propagate the default value down the element tree.

Example :

Suppose we create 1 button dynamically and we want add that button
in grid .

Grid.RowProperty,Grid.ColumnProperty is the most common dependency property.

 Before we add the button to the grid.children collection, we can set this row/col properties like:

 btn.SetValue(Grid.RowProperty, 1)

 btn.SetValue(Grid.ColumnProperty, 1)

then we just use myGrid.Children.Add(btn) and the button will get to the correct row/column "of the grid".

Samething with Canvas.

we can add button in canvas using their dependency properties like

Canvas.LeftProperty

Canvas.TopProperty

Please check below link for more information.

http://geekswithblogs.net/Silverlight2/archive/2008/12/24/understanding-dependency-properties-in-silverlight.aspx

Hope this will help you.

Thnaks.