A message bus interconnects services in a data system just like a message broker.
The difference between both is in complexity. Message buses are less complex.
They send messages to consumers only while those consumers are connected.
On the other hand, message brokers can retain messages for a consumer even if that consumer is not connected.
Message buses are also less complex than message brokers with regard to protocols and validation.
A message broker can convert messaging protocols and can validate the schema types of the transmitted message.
A message bus on the contrary doesn’t do all this. Usually, connected services in a bus all follow the same protocols.
The bus bears a lesser burden compared to the broker.