What is the difference between an event and a command message? How should each be named and routed?

A command asks one receiver to do something. An event says that something already happened.

Command, for example SendInvoice or ReserveStock:

• It is named as an order, with a verb first.
• It has exactly one handler.
• The sender expects it to be done, so the receiver can reject it.
• It usually goes to a point-to-point queue, where one consumer takes each message.

Event, for example OrderPlaced or PaymentFailed:

• It is named in the past tense, because it states a fact.
• The publisher does not know or care who listens.
• It can have zero, one or many subscribers.
• It goes to a topic or a fanout exchange, so every interested service gets its own copy.

The choice affects coupling, which means how much one service depends on another. With a command, the sender depends on the receiver being able to do the job. With events, you can add new consumers without changing the publisher. But the event's schema becomes a contract that many teams rely on, so it is hard to change.