The structure of a black hole is simple: a horizon surrounds a singularity. It should be noted that although people often refer to the horizon as "the surface of a black hole", there is nothing tangible on this "surface".

The spherical Schwarzschild black hole is the simplest black hole, which has neither rotation nor charge.

The most complex black hole is a black hole with both mass and charge, and it is still spinning. Because the strong gravity destroys all the details of the black hole, distant observers can only detect three physical quantities of the black hole at most, namely mass, charge and angular momentum representing the rotation speed of the black hole. Scientists call this situation "black hole hairless", which means that black holes have no complicated details.