The cave was originally not considered a biome as it was part of the winter, but Lapa's update showed that it is its own island so it is considered a mini biome. Currently, the cave is inside the winter biome again.
The cave has a gray floor, no minerals except diamond and amethyst. The only kind of mobs that spawn in the cave is dragon, which makes the cave a popular place as the dragons are the only way to get dragon gear.
It is recommened that you avoid the dragons unless you know what you are doing, whether you are wearing Explorer's Hat or not, dragons are still really dangerous.
The cave hosts two precious minerals, diamond and amethyst, making it a popular place. And unless you must, you should never do PvP in the cave.