A political label, used quite commonly in the United States and the United Nations, to denote an inhumanitarian act.
Chinese brutality such as the Cultural Revolution and Tiananmen Square is often denounced using this label by both the US and the UN. China, however, is merely the most obvious initiator of crimes against humanity; many countries are barred from entry into the Olympics for humanitarian reasons. (And even the US is sometimes accused of commiting crimes against humanity; for its strong-arm, police officer to the world policies such as the interference between the Serbs and Bosnians in Czechoslovakia and especially Kosovo during the
1990s.)
On the US domestic front, rape and infant mutilation are typical examples of crimes against humanity.
More lightheartedly, the expression crime against humanity is often used as hyperbole in American Generation X and Generation Y culture. It is used to indicate bad taste. Many people would say Mmmbop or Hanson in general is a crime against humanity. And most people over the age of five agree that Barney is a crime against humanity.