Here are a few definitions extracted from:
Control Council Law No. 10, Punishment of Persons Guilty of War Crimes, Crimes Against Peace and Against Humanity, December 20, 1945, 3 Official Gazette Control Council for Germany 50-55 (1946).
a) Crimes against Peace. Initiation of invasions of other countries and wars of aggression in violation of international laws and treaties, including but not limited to planning, preparation, initiation or waging a war of aggression, or a war in violation of international treaties, agreements, or assurances, or participation in a common plan or conspiracy for the accomplishment of any of the foregoing.
b) War Crimes. Atrocities or offences against persons or property, constituting violations of the laws or customs of war, including but not limited to, murder, ill treatment or deportation to slave labour or for any other purpose of civilian population from occupied territory, murder or ill treatment of prisoners of war or persons on the seas, killing of hostages, plunder of public or private property, wanton destruction of cities, towns or villages, or devastation not justified by military necessity.
c) Crimes against Humanity. Atrocities and offences, including but not limited to murder, extermination, enslavement, deportation, imprisonment, torture, rape, or other inhumane acts committed against any civilian population, or persecutions on political, racial or religious grounds whether or not in violation of the domestic laws of the country where perpetrated.
All three sections (a) (b) and (c) are recognized as a crime. These were formulated in London, in August 1945, to establish a uniform legal basis in Germany for the prosecution of offenders, not dealt with by the International Military Tribunal.