Mosquitto is an open-source message broker, which means it allows different programs to exchange information in a way they can all understand.
Information is exchanged via channels, where you publish information or subscribe to to receive information.
For example, you could have a channel for every sensor in house, organised by room:
The Raspberry Pi, receiving information from various places, puts that information on the right channel so home automation software like OpenHAB can pick it up.
/FirstFloor/Living/Temperature /FirstFloor/Living/Humidity /FirstFloor/Kitchen/Temperature etc.
First get the repository package signing key, import it into apt, and remove the key file again:
wget http://repo.mosquitto.org/debian/mosquitto-repo.gpg.key sudo apt-key add mosquitto-repo.gpg.key rm mosquitto-repo.gpg.key
Then make the mosquitto repository available to apt:
cd /etc/apt/sources.list.d/ sudo wget http://repo.mosquitto.org/debian/mosquitto-wheezy.list sudo apt-get update
Now we can install the core packages (server & clients):
sudo apt-get install mosquitto mosquitto-clients
Mosquitto is automatically started as soon as it's installed. To test, open a second terminal tab (you can do this as an extra tab in the same MobaXterm window).
Make Terminal 1 listen on the hello/world channel ("-d" means to output debug information, "-t" followed by a channel name specifies the channel):
mosquitto_sub -d -t hello/world
Have Terminal 2 publish something on the hello/world channel (same options as when subscribing, plus "-m" followed by a message):
mosquitto_pub -d -t hello/world -m "Greetings from Terminal window 2"
If all goes well, you should see the message appear in Terminal 1.