"The keystone of the law regulating conduct of hostilities is the principle of distinction, which requires parties to a conflict to distinguish at all times between combatants and civilians. Civilians and civilian objects may not be attacked, and operations may be directed against only military objectives.[32]
Military objectives are combatants and those objects which "by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage."[33] In general it is prohibited to direct attacks against what are by their nature civilian objects, such as homes and apartments, places of worship, hospitals, schools, or cultural monuments, unless they are being used for military purposes. An area of land can constitute a military objective if it fulfills the above criteria.[34]"

http://www.hrw.org/en/node/10911/section/5