While once it seemed as though the only onion options were the generic sounding yellow, white and red, each variety of onion in the market is now labeled clearly - and there are a lot of them, in addition to those three standards. But what is the difference between them? The primary difference is sweetness, with some onions, known as sweet onions, lacking the sulfuric bite that most associate with an onion.
Sweet onions usually are available in summer and have a higher water content than so-called "storage onions", which contributes to their mild flavor. It also means that they have a somewhat shorter shelf life and are more delicate, prone to bruising, than their less-sweet counterparts. They can be used interchangeably with regular onions in cooking, since they will handle almost identically, but they will impart slightly different flavors to the finished dish. Many people prefer sweet onions in dishes that call for raw onions, such as salads, or for use on burgers and hot dogs.