通过之前的学习笔记，可以知道，gazebo跟ROS是相互独立的环境。而gazebo也可以看作是一个硬件，然后要在gazebo的模型中插入相应的ROS插件才可以跟ROS进行通信，才可以实现基于gazebo的ROS仿真。
Gazebo提供了一组ROS API，允许用户修改和获取有关模拟世界各个方面的信息。
Terminologies（术语）
the pose and twist of a rigid body object is referred to as its '''state''. An object also has intrinsic （内在的） '''properties''', such as mass and friction coefficients. In Gazebo, a '''body''' refers to a rigid body, synonymous to '''link''' in the URDF context. A Gazebo '''model''' is a conglomeration of bodies connected by '''joints'''（关节）.
About the gazebo_ros_api_plugin
The gazebo_ros_api_plugin plugin, located with the gazebo_ros package, initializes （初始化） a ROS node called "gazebo". It integrates the ROS callback scheduler（回调调度程序） (message passing) with Gazebo's internal scheduler （内部调度程序） to provide the ROS interfaces described below. This ROS API enables a user to manipulate（操纵） the properties of the simulation environment over ROS, as well as spawn （产卵） and introspect （反省）on the state of models in the environment.
This plugin is only loaded with gzserver.
参考资料
