While they are both from the plant, Allium cepa, chives, and green onions are not specifically the same thing. Chives are harvested much earlier than green onions and are milder in flavor.  Green onions are harvested later, after the bulb has formed, and can be stronger in flavor. Both chives and green onions add a delicious onion-y kick to any dish, so use whichever you have on hand!