BPMN defines three main types of diagrams: Process, Collaboration, and Choreography, and in BPMN 2.0, Conversation diagrams were also introduced.
Formally, Conversation diagrams don’t present a standalone diagramming technique, but are just a particular usage of Collaboration diagrams.
Conversation diagrams represent a top-level view of BPMN collaboration diagrams. They are useful for representing process landscapes and high-level interactions between involved parties, such as representing an overview of a network of partners and how they communicate with each other.
Conversation diagrams are based on three main types of elements only: participants, conversation nodes and conversation links.
A participant is representative of a BPMN pool, and represents people, organizations or devices which are involved in a specific communication.
Participants are interrelated with Conversation nodes, and together with conversation links define conversations, which take place between two or more participants.
These conversations are related to collaboration diagrams, as a conversation node can be expanded into a series of message flows between participants.
And since participants in a conversation are actually BPMN pools, conversation diagrams can be directly linked to processes via conversation nodes.
Business Process Analysis BPMN Video