the main categories of mass atrocities

The "big four" categories of atrocity crimes are:

• genocide the intentional destruction of a people in whole or in part ("people" can be a national, ethnical, racial, or religious group)
• ethnic cleansing the systematic forced removal of a people from an area, with the intent of making the society ethnically homogenous
• crimes against humanity a widespread or systematic attack directed against a civilian population (either in times of war or peace)
• war crimes a violation of the laws of war

These are used primarily in the present day to prosecute international crimes that have happened recently. They don't attempt to categorize historical examples of mass killing, such as the Mongol conquests. (TODO: some info from here might go well at this point)

Resources:

• Global Centre for the Responsibility to Protect — Defining the Four Mass Atrocity Crimes