title: ROS Service and Message
body: <p>I am using ROS system and I am a beginner.
  I came across Service and Message (srv and msgs) in ROS. From the ros wiki, I manage to understand like msgs are used to define the type of message been passed and service is about request and response. Please correct me if I got this wrong.</p>
  
  <p>Nevertheless, I am unable to understand when to use them. I thought that maybe it would be useful if I have modules written in C++ and other Processing Modules in Python than perhaps I can use srv and msgs for communicating between the 2 modules. However, than ROS also have publisher and subscriber system which could be used instead?</p>
  
  <p>Secondly, when we use srv than only we need to define the msgs or either can be independently be used?</p>