A spring onion isn't a type of onion; rather, it's an onion that has been harvested at an immature stage, when it has just begun to form a round bulb and the top is still green. At this point it will seem sweeter than a mature onion because it hasn't yet developed its full chemical complex, including the elements that give onions their characteristic bite. Really, though, it's probably more accurate to call it milder, since it hasn't developed its full sugar yet either.
At farmers markets you will find spring onions in all sorts of varieties -- red, white and yellow. Because they are immature, the flavors are pretty much interchangeable.
How to choose: Look for onions that are fresh and glossy in appearance. The tops should be bright green and firm, not limp. Although these look a lot like the pencil-shaped green ones, they're actually a different type and can show anything from a slight bulge to a full-blown bulb.
How to store: Regular onions can be left out at room temperature for weeks, but because these onions are immature and full of moisture, they need to be kept refrigerated in a tightly sealed plastic bag. They'll last a couple of weeks.
How to prepare: Spring onions are really nice quartered lengthwise (leave the roots attached to hold them together), brushed with olive oil and grilled.