Their use, flavor, color, shape of the bulb and day length. Onions grown for storage often have a darker color with thicker skins, a more pungent flavor, and are usable for many months of the year. Fresh onions have a flavor that is milder and sweeter with a lighter color and thin skin. The flavors of onions range from sweet to pungent. Flavor is often dependent upon growing conditions.
Some soils seem to produce sweeter onions than others.
Soils high in sulfur seem to produce more pungent onions. Onions come in the colors white, yellow or red. The shape is globe or round, flattened or torpedo shaped. An interesting aspect of onions is the effect of day length on plant growth. Many plants are sensitive to the hours of daylight and darkness they receive.