The Biome Islands are large structures generated inside the End dimension very far from the Dragon Lair, depicting different cultures of the dimension.
All islands are similar to the main island in shape, but they're approximately half sized. Each island generates two types of ores - Stardust Ore which is pretty common, and Instability Orb Ore which spawns in very small clusters and is rather scarce. End Powder Ore can be also found, as it can generate anywhere in the dimension. They also generate caves which sometimes branch off, causing interesting underground systems.
The top of the island is populated with End Stone Terrain, which is biome-dependant and gives each island a characteristic look.
Island Core, or Biome Island Core is an unbreakable block spawned on the bottom of each island. Since Release 1.1.1, it uses the bedrock texture (previous version had no texture). The general purpose of the Island Core is allowing biomes to use special mechanics, such as custom mob spawning and also the infestation effect in Infested Forest Biome.
As of Pre-Release 1.2, there are 3 types of biomes. Each island has a chance of having one or more rare variations, which give unique look to the biome.
Each biome has its own page: