## TL;DR

MQTT is a lightweight publish-subscribe messaging protocol designed for constrained devices and low-bandwidth networks. Created by IBM (1999), standardized by OASIS. Used extensively in IoT: smart homes, industrial sensors, connected vehicles. QoS levels: 0 (at most once), 1 (at least once), 2 (exactly once).

## Core Explanation

Architecture: Broker (central server) + Clients (publishers and subscribers). Topics use hierarchical structure: `home/livingroom/temperature`. Retained messages: new subscribers receive last published message. Last Will and Testament (LWT): message published if client disconnects unexpectedly.

## Further Reading

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