Messages are the atomic units of information exchanged between services and between applications and services.
Messages play the role of either a command or an event. A command is a message that represents an instruction sent to a component. A command is an input message. Commands are sent directly to components. They are a unidirectional transmission from one endpoint directly to another.
An event is a message that represents the effects of processing a command. When a component processes a command, the result of that process is the writing of one or more events. Those events are the source material that event sourcing and pub/sub patterns are built upon.