MQTT is a topic based publish-subscribe messaging protocol.
Example: Client-A subscribes to ‘/some/topic’ while Client-B publishes a message to ‘/some/topic’. All clients subscribing to ‘some/topic’ receive the message published by Client-B, including Client-A.
“MQTT was designed to use minimal bandwidth and device resources. Like HTTP, MQTT’s payload is application specific, and most implementations use a custom JSON or binary format. Many IoT platforms support HTTP and MQTT as their first two inbound protocols for data.”