Botanically known as Allium porrum, leeks are a member of the Allium family of vegetables. More familiar Alliums include onions, garlic and shallots. Unlike their cousins, leeks differ in a couple of ways. In flavor, leeks are more subtle and sweeter than a standard onion. In structure, unlike most Alliums, leeks do not form bulbs.
In addition be being delicious, leeks have great health benefits. Leeks, like their Allium cousins, have been shown to improve the immune system, lower cholesterol and help fight cancer. More specifically, leeks are a good source of fiber, folic acid, vitamin B6, vitamin C, manganese and iron.
SimpleBites.net has a wonderful write-up on leeks, how to prepare them for recipes, and a few recipes that you may want to try.