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目錄是執行caikin_make install命令安裝路徑。C++版本在 include文件夾，Python 在 lib，Lisp在share文件夾。