Let's learn how two nodes communicate with each other using ROS topics. The following diagram shows how this happens:

As you can see, there are two nodes named talker and listener. The talker node publishes a string message called Hello World into a topic called /talker, while the listener node is subscribed to this topic. Let's see what happens at each stage, marked (1), (2), and (3) in the preceding diagram: 

1. Before running any nodes in ROS, we should start the ROS Master. After it has been started, it will wait for nodes. When the talker node (publisher) starts running, it will connect to the ROS Master and exchange the publishing topic details with the master. This includes the topic name, message type, and publishing node URI. The URI of the master is a global value, and all the nodes can connect to it. The master maintains the tables of the publisher connected to it. Whenever a publisher's details change, the table updates automatically.

2. When we start the listener node (subscriber), it will connect to the master and exchange the details of the node, such as the topic going to be subscribed to, its message type, and the node URI. The master also maintains a table of subscribers, similar to the publisher.
3. Whenever there is a subscriber and publisher for the same topic, the master node will exchange the publisher URI with the subscriber. This will help both nodes to connect and exchange data. After they've connected, there is no role for the master. The data is not flowing through the master; instead, the nodes are interconnected and exchange messages.

More information on nodes, their namespaces, and usage can be found here: http://wiki.ros.org/Nodes.

Now that we know the fundamentals of ROS, let's look at a few ROS client libraries.