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, because 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.