Questions about API

edit

Actually, you basically copied the answer in here as well :-)

ROS: functionality available via ROS topics and services

C++: functionality available in C++ libraries

Python: functionality available in Python modules/packages

ROS has client libraries, roscpp and rospy that allow you to use ROS functionalities in C++ and Python code, respectively.

So, you have a C++ API), to e.g. use Publishers and Subscribers, as well as a Python API. Those two are obviously different. From the outside, a topic looks, however, the same, wether it comes from a python or a C++ node. You can manipulate this e.g. using the topic_tools package, which are just ROS nodes written in either C++ or python.

However, this is only a very rough overview, don't get stuck too much on this list.