The really, really short TL;DR is "Even wars have limits" (a slogan used by the Red Cross).

The slightly longer TL;DR is that only active combatants are legitimate targets in war. Civilians, wounded or sick soldiers, soldiers who have surrendered, medical personnel and facilities, shipwrecked sailors and downed airmen cannot be targeted and must be protected. Additionally prisoners of war must be treated humanely.

There's some other important elements such as the role of the Red Cross and Red Crescent, that medical personnel and facilities cannot be used for military purposes etc.

The International Committee of the Red Cross has some excellent material on the Geneva Conventions.