Today I worked with the BizTalk Shapes Call and Start Orchestrations. In German they have the silly names "Aufruforchestration" and "Start Orchestration" which do not express their purpose. I found to really good blog entries about this topic:
http://dallas.sark.com/SarkBlog/mholdorf/archive/2005/10/12/2183.aspx
http://www.topxml.com/BizTalk-Rule-Engine/re-2671_Processes-for-invoking-BizTalk-orchestrations-both-synchronously-and-asynchronously.aspx
Also a fine example from Scott Woodgate exists. (See at end of second link)
Short hint: If you call an orchestration from another project you have to set the Type Modifier Property to public. Otherwise the orchestration will not show up in the dialog where to choose the orchestration you call.