In a recent discussion, Chowhound users asked a question: Are there subtle differences between green onions and scallions? And what about leeks?
The names green onion and scallion are used interchangeably these days, biondanonima explains, though originally there may have been some regional variations. Still, some Chowhounds think of scallions as the young onion shoots with white tips (sort of like miniature leeks), while green onions are more bulbous. But according to this CHOW Nagging Question column, in fact there's no difference.
Leeks, on the other hand, are a completely different plant, sunshine842 says. They have a milder, sweeter flavor than green onions, and respond well to braising, sautéing, baking, and other methods where green onions might turn slimy, biondanonima notes. Interestingly, cresyd thinks green onions work best cooked either for a very short or a very long time.
Photo of CHOW's Korean Scallion Salad by Chris Rochelle / CHOW.com