Realistic Terrain Generation (RTG) is a mod that adds a new world type which generates realistic terrain. It doesn't add new blocks. It doesn't add new mobs. It doesn't even add new biomes. It simply generates more realistic-looking terrain for existing Overworld biomes (including those added by other mods).
For more information, please visit the official RTG thread on the Minecraft Forum: http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2524489-realistic-terrain-generation-rtg-realistic-biomes