Most of the time, scallions and green onions are the exact same thing, but there can be a technical difference between the two. This difference can be boiled down to what type of species the green onion is grown from (a bulb-producing onion or a non bulb-producing onion). More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.