MQTT is a lightweight protocol designed for the Internet of Things (IoT). It is a publish/subscribe protocol that is ideal for low-powered devices with limited bandwidth and unreliable networks. It is a simple protocol that is easy to implement and is designed to be used in constrained environments.
CoAP is a specialized web transfer protocol for use with constrained nodes and networks in the IoT. It is designed to be used in machine-to-machine (M2M) applications such as smart energy and building automation. CoAP is more complex than MQTT and has more features such as multicast support, secure communication, and resource discovery.
AMQP is an open standard application layer protocol for message-oriented middleware. It is designed to provide reliable, secure, and asynchronous message-based communication between distributed applications. It is more complex than MQTT and CoAP and is designed for enterprise-level applications.
In summary, MQTT is a lightweight protocol designed for the IoT that is easy to implement and is ideal for low-powered devices. CoAP is a specialized web transfer protocol designed for M2M applications. AMQP is an open standard application layer protocol designed for enterprise-level applications.