Suppose I throw a computer into a black hole. This computer's hard disc contains a great deal of usefull (and useless) information. Once the computer falls below the "point of no return", the information on this hard disc is lost for ever to the outside world. This is not a problem since in principle, if I wanted it badly enough, I can fall down the black hole after it, and retrieve it. But now we know that black holes are not stable: they evaporate. Moreover, this evaporation occurs due to microscopic processes just outside the "horizon", and it cannot know about anything what has already fallen through the horizon. Thus it cannot contain any information about what is inside: the radiation that it emits carries no information. We call it pure heat, or thermal radiation. The second picture indicates the black hole after it has evaporated a little: the surrounding universe is a bit hotter, and the black hole, which has lost energy, is correspondingly smaller. If we follow this process to its logical conclusion, what we have at the end is no black hole, only thermal radiation filling the universe. The information on the hard disc has irrevocably disappeared along with the black hole, and there is no way to retrieve it, even in principle. In physics, such information loss is unacceptable: it means among other things that the future cannot be predicted by knowing the past. There is no apparent correlation between the thermal radiation that fills the Universe, and the state of the Universe (i.e. the hard disc) before it was thrown into the black hole.