Green onions and scallions refer to the same plant, a variety of allium, or onion, species that lacks a fully developed root bulb. In mid-Atlantic coastal states they are called scallions, while mostly everywhere else they are called green onions.
The plant consists of long, green, hollow leaves and a white stem. While most of the time only the green leaves are consumed, the whole plant is edible.
Scallions or green onions can be used raw or cooked. Their flavor is similar to onions but considerably milder, and they are usually available in grocery stores throughout the entire year. They are common in Asian cuisine and are used in salads, salsas, soups, rice and other dishes.