I have world file to launch then should i include my urdf of robot model inside the world too?

edit

Actually, I'm creating a robot on my own .Now, what happened was when I launched the urdf file using the launch something like this:

<launch>
<param name="robot_description" command="$(find xacro)/xacro --inorder '$(find 2wr)/urdf/two.xacro'" />
</launch>

The world in gazebo doesn't load with robot model. Now I was confused so I went on to add URDF of the robot into the world sdf and now robot appears in gazebo. Wrote all the that I wrote in URDF file inside this "modeL" tag.

<model name='2wr'>
<link name........
..............</model>

So my question is should we have to describe the structure of robots both in URDF as well as SDF(i.e inside the WORLD file)??