Green beans are the seed-filled pods of the legume family which, properly speaking, occur in the early days of the pod's life, up until about a week. At this young age the pod of the bean is tender enough to be eaten, and the seeds therein are mere nubbins, small enough to be ignored. Bean pods at this stage generally snap when folded in half, and hence beans with edible pods are sometimes called snap beans or just edible pod beans. As the bean matures, the pod becomes tough and leathery; at this stage the pod is usually discarded, and only the seeds inside are eaten. Still later the seed dried out, and you've got dried beans.
Edible pod beans include the rounded, skinny French bean or haricot vert; the rounded, thicker string bean, so called because of the tough string that used to run along its length but which has now been mostly bred out; the broad, flat runner bean; the long, skinny Asian yard long green bean; and the weird and wonderful wing bean. Not all edible pod beans are actually green; some are yellow (wax beans) or purple. But whatever their colour, be sure to choose crisp, bright-coloured, blemish-free beans and store them in a plastic bag in the fridge; use them within a few days of buying or, if you're lucky, picking. Edible pod beans are excellent steamed, blanched, or stir fried. (Ouroboros correctly reminds me that many edible pod peas like snow peas and sugar snap peas are also of the legume family, and should be bought, stored, and cooked in similar ways to edible pod beans.)
A word about flatulence: it's caused by complex sugars that are only present in dried beans. Edible pod beans will not cause this particular problem, so eat them, eat them often, eat them well.