What is the difference between a topic and a message?

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Topic is a channel - you are correct, but additionally to a name topic can transmit only the messages of a particular type. Thus, a message to be sent across this channel should have the same type.

Imagine that you have a square pipe (this will be a channel-topic) and a set of cubes (these are messages). All the cubes have same dimensions, but different colours, and even some of them have letters engraved. So the 'square' will be the type of a message and colours and letters will be the contents of that message. That means you cannot send (publish) or receive (subscribe to) 'circle' and 'triangle' messages on that topic.

We have messages, and every message has a message type.

Now, every defined topic should have one of those types, so publishers/subscribers to the topic can send/receive messages, of different values but of the that same type, through the topic.