There's a great series of posts on BPMN diagrams at Tyner Blain.
What are BPMN diagrams? They are Business Process Modeling (as-is, to-be, etc) diagrams that conform to the Business Process Modeling Notation Standard. You didn't know there was an official standard? Neither did I. There are even Visio Stencils available.
I look forward to reading the rest of this series.