The next advanced layout element in Xaml is the stack panel. The StackPanel lays out it’s children either horizontally or vertically without having to define the exact location in the visual tree in Xaml. Take the following listing example of a stack panel...
read the rest at:
http://hackingsilverlight.blogspot.com/2008/02/using-silverlight-20-xaml-stackpanel.html
The Grid is basically just that a grid of a set number of colums and rows. Elements in the grid must then be placed into a cell in the grid by setting the Grid.Column and Grid.Row property of the element in questions. To start using a grid you first need to create a grid node. Here is a simple example...
http://hackingsilverlight.blogspot.com/2008/02/using-silverlight-20-xaml-grid-element.html