Quote:
A flat or undulating "ground", along with a "horizon" which is usually done as a "billboard". Look up the term "billboard". Clearly, the horizon should be far enough from the player that it's not "obvious" as a billboard.
Horizons are not billboards. The terrain is some type of mesh, the 'horizon' is usually a skydome or skybox, tree's and other vegetation are normally 2 billboards that cross each other at right angles but recently they have gotten more complex than that, and clouds are normally billboards or texture splatted billboard textures that are rendered.