This is a very easy way to animate the grass of a scene in a realistic way. You render the grass the usual way with intersecting quads distributed randomly on the terrain you want the grass on. Then in the vertex shader, the top two vertices are displaced with the combination of four sinus waves. This gives a waving grass.

The result might look like this (this really depends on the quality of the textures used for grass).

In the original paper, you can find a source code for the shaders and full description of the technique.
http://ati.amd.com/developer/shaderx/shaderx_animatedgrass.pdf