Green onions are young shoots of bulb onions, and are milder tasting than large bulb onions.
They have a small, not fully developed white bulb end with long green stalks. Both parts are edible.
Scallions are considered younger than a green onion because they should not have a bulb, while green onions should have a miniature bulb.
Negi or naganegi onions, which translates into "long onions" are another variety. Plural: green onions