I am running a MQTT broker on a server that i have, I am using Mosquitto MQTT Broker.
mcGateway is connected to that broker and my mcModules is publishing information to that broker.
Example the current temperature, battery voltage on the modules and things like that and its easy to add more functions to the modules, monitor open windows/doors.
maybe you have seen my example in the forum about connecting to a touchscreen.
One of the reasons that i like the MQTT is that it is very simple to connect things to it, create a homepage that subscribes on some topics and displays the current temperatures.
If you connect a relay to a mcModule then you could turn on / off that relay from the same homepage.
Very simple to send/receive information and let many different things share information.
I am using MQTTlens just to see what's going on when I am testing some new code that publish to the broker, I could also from MQTTLens publish information to the broker, so it's most for debugging.
I could recommend reading the different parts of this blog
http://www.hivemq.com/blog/mqtt-essentials-part-1-introducing-mqtt just to get started with MQTT.