I don't what ROS is introduced, the concept of a lot of books have detailed instructions, once complete white (must be a c + + based), I will only in the language of the most popular here to help you learn about understanding ROS,
Programming learning ROS the most important thing is to practice! Premise is to understand some ROS knowledge structure, the following I will be a small white of summary only commonly, be sure to completely write their own code, even if you back down to the template, (as write programs to write after so many tips)
ROS communication mechanisms 1: the topic (topic) : must have the subscriber (the listener) and the publisher (talker), create a publisher need to specify the data type, and the subscriber does not need (but) specified in the callback function,
ROS communication mechanisms 2: service (serve) : must have the client (client) and server (server), same as above mentioned, but this kind of communication is synchronous communication, topic is asynchronous communication, if not understand synchronous asynchronous please to understand again after baidu,
ROS communication mechanism 3: tf tree feature pack (although this does not belong to the communication mechanism, but in the later you will find that the tf trees are of great use, similar to the inside of the programming language global variables, using it to include the header file tf/tf, h), it must be understood, even if you may tf, you put the things hanging in a tree, then a lot of people can pick what you got what you,

Actually these communication mode is nothing but data communication between nodes and node, the node is a. CPP or. Py files, for example: a. CPP file is responsible for retrieving data, another is responsible for handling the data, natural need to transmit data, I also feel simple data transfer, previously compiled C is often used, in ROS make gaudy, but who call ROS is cutting edge, we also recognize the beep,