"Green garlic is harvested before the garlic cloves have begun to develop. The resulting plant looks almost like a scallion. It is milder in garlic flavor and when it is cooked, it sweetens and gets a unique complex flavor which needs minimal seasoning." (http://www.uncoveringfood.com/2010/04/creamy-green-garlic-and-fennel-soup.html)
"Green garlic and garlic scapes add delicate garlicky flavor to spring dishes. These hallmarks of spring are available starting in March in warmer climates through June in cooler ones.
Green garlic is simply immature garlic and looks like a slightly overgrown scallion or green onion. They are pulled by growers when thinning crops and, increasingly, grown as a crop in their own right. Look for specimens with fresh green tops (no dried ends or soggy leaves). To use, trim off root ends and any tough part of the green leaves. Chop or slice white, light green, and the first few inches of the dark green leaves (as long as they are tender). Use as you would green onions or garlic, noting that it is stronger than the former but milder than the latter.
Garlic scapes are the curled flower stalks of hardnecked garlic varieties grown in colder climates. They are cut off by growers to encourage better bulb growth and available through early summer in colder growing regions. Crunchy, with a mild garlic flavor, garlic scapes are great in salads and stir-frys.
Demand for both green garlic and garlic scapes is growing as more restaurants put them on their menus and people discover their mild but distinct flavor. They are still mainly only available at farmers markets and farm stands, but you might see them at gourmet markets and even large supermarket produce sections in some cites." (http://localfoods.about.com/od/greengarlicgarlicscape/a/aboutgrgarlics.htm)