Choreography Diagrams specify the way business participants coordinate their interactions. They can also be seen as a business contract between participants, where the focus lies on the exchange of information (Messages) between the participants.
Business contracts are often in the form of a purchase order sent to a supplier, the confirmation by the supplier to process the order, then the fulfilling of the order. Choreographies also have Activities ordered by Sequence Flows.
Activities comprise of one or more interactions between the various participants. Interactions are often called Message Exchange Patterns (MEP). A MEP is the "Activity" of a choreography, and can also be called a Choreography Task.