Yellow Onion: This is what most cooks reach for when a recipe simply calls for “onion.” It’s higher in sulfur than the white onion, so it has a more complex flavor. The sulfur, unfortunately, is also what makes you cry when you cut into it. Yellow onions turn a rich brown and become sweeter and milder when cooked. Many people find them too pungent to eat raw.
Sweet Onion: These onions are mild and crisp, so they’re the onions of choice for slicing raw on burgers and sandwiches. They can be lightly cooked, too, though they’re not as pungent and flavorful as yellow onions. There are several different varieties, often named after the region in which they’re grown. The most popular include Vidalia, Walla Walla, Sweet Imperial, Texas Spring Sweet, Texas 1015Y, Carzalia Sweet, Oso Sweet, Arizona, Granex, and Maui. They’re usually available from March through August, though some producers extend the season by storing them in a low-oxygen environment. Sweet onions are usually larger than storage onions. They also have a higher water content, so they don’t keep as well. (Adapted from foodsubs)
~ Chef Sandy of “The Kosher Tomato”