There is an excellent article on Reporting Services Architecture for Microsoft SQL Server 2005 Reporting Services (SSRS) at csharp-online.net. There is a great diagram on the first page. Reading the article from front to back is a little difficult, as it is broken up in small pieces, but the navigation menu to the right of the text is helpful for moving around when tracking down references. The article presented here is an extract from a book by Michael Lisin and Jim Joseph.