Chaga goes by several names. The botanical name for the fungus is Inonotus obliquus. The Chinese call it Hua Jie Kong Jun or Bai Hua Rong. While it is often referred to as a mushroom, it grows as a dense black colored mass on the outside of trees. Most commonly it infects birch trees.
Chaga does not form a fruiting body the way most mushrooms do, but rather a mass of mycelia that incorporates decaying bits of the birch tree it grows on. Mycelia are microscopic vegetative fungi parts that combine to create a whole growing organism.