ROS actions follow a client-server concept, as shown in the following diagram. The client is responsible for sending control signals, whereas the server is responsible for listening to those control signals and providing the necessary feedback:

The client node and the server node communicate via the ROS action protocol, which is built on top of ROS messages. The client node will send a goal or a list of goals or cancel all the goals, whereas a server application would return the status of that goal, the result upon the completion of a goal, and feedback as periodic information about the goal being achieved or cancelled. Let's look at this in more detail by looking at an example.