Biomes are what the developers refer to as the different themed lands, or different “skin packs” for the game. In the current version of K2C, we have two biomes: the default biome and the shogun biome. Biomes can only be selected on the loading screen when you first start a save. More biomes will be added later.
The default biome is the normal biome for Kingdom: Two Crowns. The only difference gameplay-wise is that you can hire pikemen.
The shogun biome is a biome themed after feudal japan, available as a free DLC. Squires become ronin, knights become samurai, and coins become mon. The only differences gameplay-wise are that the forests are more densely packed with bamboo (trees), and you cannot hire pikemen, but can hire ninjas instead.
The game rotates through 4 seasons, each having their own effect on your kingdom. A season lasts for 16 days, with a full rotation of all 4 seasons taking a total of 64 days. The seasonal rotation persists between islands, and will only reset to spring if the player gets a game over. Blood moons will occur more frequently the more seasons that pass.
Spring is the starting season. During Spring, grass will begin to regrow, spawning anywhere it had been before winter came. This will allow rabbits and deer to spawn once again, and farms will also resume functionality.
In terms of gameplay, the only difference between Spring and Summer is that grass no longer spawns during Summer; it can only expand from already established patches of grass.
Fall and Summer are identical in terms of gameplay. (unconfirmed)
During Winter, all grass is gone, no more deer spawn, and farms are frozen over, leaving you with no easy way to make money for the duration of winter. The only sure form of income is by having your pikemen fish, which is exclusive to the default biome. Your best bet is to prepare to last it out beforehand.