In relation to Heightmap, Voxel and Polygon (geometry) terrains:

1. What are the main differences between all these three?
2. Can you form a "smooth" terrain with Voxels, I mean, can you for example get a smooth mountain with Voxels, or Voxels are limited to cubes?
3. Performance wise, a world 2000x2000 units, what would be faster Heightmap terrain, Voxel terrain or Polygon based, geometry terrain? (Assuming that there is "reasonable" performance gains/optimization done for rendering for every of possibilities)
4. Are there any more techniques used for terrain creation?
5. Any good titles representing each of types?

P.S. Polygon based terrain should be fully traingulated, no squareish stuff.