The Kerr
metric is the
solution to
Einstein's equations for a non-
spherically symmetric vacuum, that is a
rotating black hole. As well as an
event horizon, it also possesses an
ergo surface, which is a
surface within which it is not possible to remain at a
constant angle; in other words, within the
ergo surface, one must
rotate about the
origin. Effectively, the
rotating black hole is dragging
spacetime along with it, forcing
particles falling into it to
rotate.
The Kerr metric is exceptionally important, as it can be shown that all black holes eventually become Kerr.