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msg : msg files are simple text files that describe the fields of a ROS message. They are used to generate source code for messages in different languages.
srv : an srv file describes a service. It is composed of two parts: a request and a response.

devel目录是执行caikin_make install命令安装路径。C++版本在 include文件夹，Python 在 lib，Lisp在share文件夹。