I am new to ROS and am wondering about the difference between launch script and launch node?

edit

Launch files allow you to run multiple scripts and define input variables, and set default values. Personally, I would find it hard to remember which combination of nodes and parameters I need to run without having it consolidated into a launch file.

Also, workspaces may contain C++ and python code, but you can run either with roslaunch.

you mean i can put file.py in a package and run it with roslaunch just like for launch file

Yes. Here is an example of a launch file calling other launch files as well as two scripts:

<launch>

  <include file="$(find turtlebot_bringup)/launch/3dsensor.launch" >
  </include>

  <include file="$(find usb_cam)/launch/usb_cam-test.launch" >
  </include>

  <include file="$(find my_fruit_finder)/launch/my_fruit_finder.launch" >
  </include>

  <include file="$(find my_moveit_config2)/launch/my_planning_execution.launch">
  </include>

  <node pkg="robotiq_2f_gripper_control" type="Robotiq2FGripperRtuNode.py" name="robotiq2FGripper"  args="/tmp/ttyUR">
  </node> 

 <node pkg="my_robot" type="execution_commander.py" name="move_group_python_interface"  output="screen">
  </node>

</launch>

Thank you very much, by the way I want to make a simulation example that controls turtlebot to move along a diamond in Rviz. What steps should I take?

I think you should open a new question for this. I am not a moderator, but you would likely find more feedback and make it easier for others to find your answer in the future :)

That said, I would probably start off with exploring a ROS package for the turtlebot and look into Gazebo