They are all the same family of pea, but they are sold at different stages of maturity. Snow peas are flat with barely formed peas inside. Sugarsnaps contain more developed, plump peas, but are not as mature as garden peas. Both snow peas and sugarsnap peas are grown for their crisp, sweet edible pods.
For peas eaten whole, a good helping is about 125g for each person. Garden peas should yield just under half their unshelled weight. For two people, allow about 500g of unshelled peas, unless you are serving them with other vegetables or using them in a salad.
** Vegetarian Recipes