Biomes are what the devs refer to as the different themed lands, or different “skin packs” for the game. In the current iteration 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. The only difference gameplay-wise is that 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 gets longer as time goes on, making long-term staying at a singular island very difficult to even the most skilled players. Blood moons will occur more frequently during the drawn-out seasons.
Spring is the starting season. During Spring, grass will begin to regrow, spawning anywhere it can on open land. This will allow rabbits and deer to spawn once again, and farms will also become functional again.
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.
During Fall, existing grass will begin to decay.
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. Your best bet is to prepare to last it out beforehand.