I read this interesting explanation (and sort of roadmap) for ASP.NET. It's definately a technical read, but does give an overview of what ASP.NET is.

So if you want to find out more about ASP.NET Web Forms, ASP.NET AJAX, ASP.NET MVC, Dynamic Data and Silverlight; go here: http://weblogs.asp.net/paullitwin/archive/2009/03/30/the-four-pillars-of-asp-net.aspx