Add elements (obstacles, walls, etc.) in a URDF file

I've just started to learn how to implement an URDF. To do it, I'm following the [tutorials](http://wiki.ros.org/urdf/Tutorials) about URDF in ROS Wiki. But, those tutorials only talk about how to define a robot, but If I want to add obstacles or walls to show them in Gazebo. How can I do it? Maybe I can add those things inside the world definition.