Garlic originated in either India or Asia (the exact origin is unknown). Each year the crop must be thinned to make room for the mature garlic crop. These immature garlic bulbs with their long greens actually became a secondary crop for the farmers.
Today, garlic is grown in all but the most Northern, cold locations, worldwide. The practice of preserving the green garlic thinnings has continued so they are available in farmer's markets each spring.
In general green garlic can be used in any recipe that calls for mature garlic. The end product will produce a delicate flavor, more mild than matured raw garlic.