The leek is a vegetable, a cultivar of Allium ampeloprasum, the broadleaf wild leek.
Like garlic and onion, leeks are a member of the allium family, but have their own distinct flavour – quite harsh when raw (only very young leeks are eaten this way) but, when cooked, very delicate, like a mild onion but with a hint of sweetness. Two thirds of their length is white and firm, and this is the part that is mainly eaten. The rest of the third is made up of the leaves (flags), most of which is discarded.
Souce: BBC GoodFood