You're reading the documentation for a version of ROS 2 that has reached its EOL (end-of-life), and is no longer officially supported. If you want up-to-date information, please have a look at Humble.
ROS 2 Client Interfaces (Client Libraries)
ROS Client Interfaces, a.k.a. client libraries, are user facing interfaces which provide the high level functionality and are built on top of the rcl and rosidl APIs.
The rclcpp Package
The ROS Client Library for C++ (rclcpp) is the user facing, C++ idiomatic interface which provides all of the ROS client functionality like creating nodes, publisher, and subscribers.
rclcpp builds on top of rcl and the rosidl API, and it is designed to be used with the C++ messages generated by rosidl_generator_cpp.
rclcpp makes use of all the features of C++ and C++11 to make the interface as easy to use as possible, but since it reuses the implementation in rcl it is able maintain a consistent behavior with the other client libraries that use the rcl API.
The rclcpp repository is located on GitHub at ros2/rclcpp and contains the package rclcpp.
The generated API documentation is here:
The rclpy Package
The ROS Client Library for Python (rclpy) is the Python counterpart to the C++ client library.
Like the C++ client library, rclpy also builds on top of the rcl C API for its implementation.
The interface provides an idiomatic Python experience which uses native Python types and patterns like lists and context objects, but by using the rcl API in the implementation it stays consistent with the other client libraries in terms of feature parity and behavior.
In addition to providing Python idiomatic bindings around the rcl API and Python classes for each message, the Python client library takes care of the execution model, using threading.Thread or similar to run the functions in the rcl API.
Like C++ it generates custom Python code for each ROS message that the user interacts with, but unlike C++ it eventually converts the native Python message object into the C version of the message.
All operations happen on the Python version of the messages until they need to be passed into the rcl layer, at which point they are converted into the plain C version of the message so it can be passed into the rcl C API.
This is avoided if possible when communicating between publishers and subscribers in the same process to cut down on the conversion into and out of Python.
The rclpy repository is located on GitHub at ros2/rclpy and contains the package rclpy.
The generated API documentation is here: