Onions can be divided into two categories: spring/summer fresh onions and fall/winter storage onions.
Spring/summer Fresh Onions Spring/summer fresh onions are available in yellow, red and white throughout their season, March through August. Fresh onions can be identified by their thin, light-colored skin. Because they have a higher water content, they are typically sweeter and milder than storage onions. This higher water content also makes them more susceptible to bruising.
With its delicate taste, the spring/summer onion is an ideal choice for salads and other fresh and lightly-cooked dishes.
Fall/winter Storage Onions Fall/winter storage onions are available August through April. Also available in yellow, red and white, storage onions have multiple layers of thick, dark, papery skin. Storage onions have an intense flavor and a higher percentage of solids.
Storage onions are the best choice for savory dishes that require longer cooking times or more flavor.
When purchasing onions, look for dry outer skins free of spots or blemished. The onion should be heavy for its size with no scent.
One medium onion equals about 1 cup chopped onion.
Store your onions in a cool, dry, ventilated place - not in the refrigerator. Do not store whole onions in plastic bags. Lack of air movement reduces storage life. Chopped or sliced onions can be stored in a sealed container in your refrigerator for up to 7 days.
Source: National Onion Association