RE: Layers
Hi Petra
You can use layers to organize related shapes on a drawing page. A layer is a named category of shapes. By assigning shapes to different layers, you can selectively view, print, color, and lock different categories of shapes, as well as control whether you can snap to or glue to shapes on a layer.
On the View menu, click Layer Properties, and then click New.
Type a name for the layer, and then click OK.
In the Layer Properties dialog box, click to place a check mark in the column for properties that you want the layer to have, if they are not already checked.
Note - The layer is created for the current page, not for all pages in the document.
Also see the following link on the microsoft website for some comprehensive info on layers:
http://office.microsoft.com/en-us/visio/CH010408701033.aspx
Regards
Richard