What are white holes? Is there any other kinds of "holes" in the universe?

astronomyblog:

Well, black holes were predicted by Einstein’s theory of relativity and actually discovered over the last few decades.

In Astrophysics, White Hole is a theoretical object predicted by the theory of relativity that functions as a time-inverted black hole. Because a black hole is a region in space where nothing can escape, the time-inverted version of the white hole is a region in space where nothing can fall. Instead of pulling the material in, a white hole would blow the material into space.

White holes appear as part of one of Karl Schwarzschild’s solutions to Einstein’s general relativity equations in which a Schwarzschild worm hole is described. At one end of the wormhole is a black hole sucking matter, light, and all, and at the other end a white hole, creating / expelling matter and light.

Even though this may imply that black holes in our universe can connect to white holes elsewhere, it is not considered possible to exist for two reasons. First, because Schwarzschild’s wormholes are unstable, disconnecting as soon as they form. Secondly, Schwarzschild’s earthworm holes are a valid solution only as long as no matter interacts with the hole.

The existence of white holes disconnected from black holes is doubtful, since such hypotheses seem to violate the second law of thermodynamics. 

That is, white holes are mathematically viable physical entities, which is not to say that they exist in nature.