Well, I think that depends on what kind of terrain are you looking for. If you just want some hills and stuff like that, you can just paint them (I think the tool is called artisan) onto the nurbs or poly plane. I think another way could also be by makeing a hight/displacement map (first you make a poly plane, assign a displacement map to it, and then convert displacement to polys). But i'm not a pro when it comes to making terrains.
Hope this can help you.