strictly speaking billboards are the combination of a rectangle polygon (usually) in the 3D scene and a picture image usually from a file (often with alpha masking). In this context, they provide the pictures. The rectangle part is up to you, usually, but it is conceivable that they include a sample uv-mapped and textured rectangular polygon (or two triangles covering the same).

Here's another example:
http://thebest3d.com/pdp/tutorials/blender/criss/09_bridgeoflove.jpg

the various tufts of tall grass are done on billboards.