The Tarp Shelter is probably the most basic structure you can build. You need only a tarp or sheet of plastic and some rope or cord. As you can see in the picture below, the cord has been tied between two trees and the plastic sheet draped over it. Rocks are used to hold down the corners, as it is always best to not puncture it by using stakes. The material is too thin to be punctured and still hold up to use without tearing.
Notice that I have depicted it as being erected over a slight depression in the ground. This allows for a little more interior space, and room for a nice evergreen sprig mattress to keep one off of the ground. If you are going to do this, make sure that the depression is not located in such a way as to collect water. If necessary, build small embankments just inside the tent at either end in order to dam off any water that may trickle your way.