Modifying navigation in ASP.NET and SharePoint is relatively straightforward when you understand the different rendering methods for controls.
ASP.NET has controls that enable rapid development of navigation in Web sites. Those controls can easily be modified in SharePoint Designer.
The navigation of SharePoint sites can be controlled via the browser or via SharePoint Designer.
When you understand what CSS is used for the rendering, customization of the SharePoint navigation control is as simple as modifying the default ASP.NET menu control.