There are two different product lines here. The billboards are flat transmapped planes, much like the planes in the various environment sets, but of higher quality. They look great for medium and long range shots.

The other product is Xfrog itself which is an excellent plant and tree creator. It creates true 3d trees. A word of warning though it doesn't work well with poser because the models it creates are gigantic like almost 10 million polys gigantic. In order to reduce the number you have to have the Xfrog Tune program which adds over a $100 to the price tag.