Question

Is there a difference between a message handler and a command handler?
Or is there a difference between a command and a message?

Edit: There is 3rd candiate called command message … ugh.

Answer 1

Actually, the difference is not in structure nor implementation, it is rather a conceptual one – both of these structures are to represent different concepts.

Message – a structure that transports some kind of information

Command – a structure that triggers some kind of action

That said, a Command is a kind of Message, as it transports information about action to invoke at least (and in most cases additional parameters as well).
In terms of implementation, both MessageHandlers and CommandHandlers look very similar and depend on what do you want to do with the information.

Examples:

UserRegisteredMessage (userName) – a message that informs about registered used in a system, a handler can for display this information on UI

RegisterUserCommand(userName) – a command that instructs our system to register a user, this may involve some kind of additional action (like checks for uniqueness) and can can fail.