Biomes are regions in a Minecraft world with varying geographical features, flora, heights, temperatures, humidity ratings, and sky and foliage colors -

A river bisecting a jungle biome. At the top right is a forest -

Biomes are climatically and geographically defined as contiguous areas with similar climatic conditions on the Earth, such as communities of plants, animals, soil organisms, and viruses and are often referred to as ecosystems.

Biomes separate every generated world into different environments, paralleling the real world; examples of biomes include the forest, jungle, desert and ice plains - wikipedia