57 new biomes (50 surface, 7 caves) appear in newly explored chunks. Existing areas stay unchanged and the terrain blends seamlessly.
Every biome has its own grass, foliage, water, sky and fog colours, particles, music, trees and animals.
Find them: mp;2/locate biome lt_bi:t;name>mp;r (e.g. lt_bi:ahornwald) or mp;2/biom locate t;name>mp;r. F3 shows the biome.
Biomes appear where the climate fits: maples in cool forests, salt flats in flat dry deserts, ice caves under snow.