They are called table grapes, and can be one of many varietals. Thompson seedless is the classic green grape. Table grapes are grown to create big berries and typically have much lower sugar content, which make them better for eating when harvested, and worse for wine production. Check out these sources:
http://www.fruitandveggieguru.com/Grapes.html?pccid=54&tabid=70
http://www.tablegrape.com/