A scallion is young immature onion on which the bulb has not formed fully. They are also know as green onions or spring onions. The have a milder flavor then an onion that has been allowed to grow to maturity with a full rounded bulb. You can use the white ends and most of the green tops. Just follow your recipe as some specify the white ends only. Scallions are also best served raw as they loose their flavor when cooked.