Sometimes, links on the Microsoft and MSDN sites disappear or move, so I made a copy of the architecture diagram from BizTalk. It hasn’t been updated since then; while new features have been added, the architecture really hasn’t changed.
BizTalk Server 2009 Runtime Architecture Poster
NOTE: Download the above file (right-click Save-AS), then open with Adobe Acrobat PDF reader, and zoom in to the desired section. This is a wall-size poster.
Below are some other common architecture diagrams that appear in the above poster in a slightly different form: