First of all, Intentional targeting & unlawful killing of unarmed civilians. The bombing of civilian areas in conflicts such as the Syrian Civil War, where airstrikes have frequently hit residential neighborhoods is considered as a war crime.
The civilian population is non-combatant formed of the most vulnerable including children and the elderly that must be protected by human morality.
Then, we have Attacks on Protected Objects. The goal here is to protect cultural and indispensable sites to a given population such as schools, hospitals, historic sites and places of worship.
The destruction of historic and cultural sites in Syria, including the ancient city of Palmyra, by the Islamic State (ISIS) is a war crime.