Heres an article I found on onions:
Yellow Storage Onions?
The most common cooking onions, yellow storage onions are sold in red net bags.? They are hot and usually will make you cry.? They are a dry onion, with a heavy brown wrapper and little crispness.? Their heat disappears with cooking.? This onion is the workhorse of cookery?good in any heated dish or any other dish in which subtlety is not an issue.
White Storage Onions
These onions are hot, but with a slightly sharper, cleaner flavor than yellows.? They also tend to have a slightly shorter shelf life because they lack the pigment that affords them protection against mold and because they have proportionally more water than the yellows.? They are most commonly used in Mexican cuisine.
Spanish Onions
Spanish onions are a large yellow storage onion, as round as a globe.? They usually have a slightly higher water content and so generally are less hot, sweeter, somewhat crisper and more perishable then the storage onion.? They have some heat?just enough to let you know that they mean business.? Spanish onions can substitute for specialty sweet onions out of season.
Red Onions
Red onions are similar to Spanish onions in their characteristics:? Their flavor is sharp, sweet and pungent; their texture is a bit coarser with a very thick wrapper.? Uncooked, red onions make a handsome addition to salads of all types.? However, when cooked, these onions lose some of their color.
Boiling Onions
Boiling onions are very small yellow or white storage onions, usually about two inches in diameter.? They will be hot to the taste before cooking and are best when left whole and boiled or simmered in a stew.
Pearl Onions
Pearl onions are between 1 inch and 1-1/4 inches in diameter with a thin, white wrapper.? They are crisp, with a surprising sweetness and only after chewing does a little sharpness kick in.? Pearls are very good when marinated or pickled.
Specialty Sweet Onions
Specialty sweet onions are very high in water content, low in heat and high in sweetness.? They are extremely crisp and fairly perishable.? These are the onions that most often are sold under a regional name (Maui, Vidalia, Walla Walla and have a limited season.? Usually yellow, most are shaped like globes or slightly flattened globes.? These onions are excellent raw, delicate when cooked and they caramelize nicely.? Try them for your next onion rings.
Leeks
Leeks have a tough green top that is discarded.? In flavor, the garden-variety leek is like an offbeat onion, very hot, coarse and chewy.? Leeks are best cooked.? When cooked, leeks develop a warm, oniony flavor.? When chopped and sauteed, they become almost buttery in texture.
Scallions
Scallions have a white bulb that is mild in flavor and enjoyable both cooked and raw.? Scallions are a common ingredient in Chinese cuisine; the bulb is cooked to add flavor, the raw greens tops are chopped and sprinkled on at the end as a garnish.
Shallots
Shallots, which are like tender, delicate onions in taste and aroma, come in a variety of sizes.? They make a superb base for sauces and a splendid addition to omelets.? Cooked whole, they hold their shape and caramelize beautifully, making them an excellent addition to anything braised or roasted.? Like the leek, the shallot is not for eating raw.