These members of the onion family, which originated in Asia or the Mediterranean, are similar to green onions in that both are elongated and don't have a round bulb, but leeks are much thicker and their dark blue-green tops are wide and coarse. They have always been very popular in Europe, particularly in France where they were referred to as "poor man's asparagus".
In Wales leeks have been part of the national emblem ever since Welsh soldiers tied them to their helmets during battle to distinguish themselves from the enemy.
Try to buy leeks in bunches tied with string, with as much of the green part left on (leeks hate plastic bags), and wash carefully right before using to remove the grit from all its inner parts. Leeks marry very well with potatoes in gratins or with chicken in the famous cock-a-leekie soup; in savory tarts, braised, or maybe Potage Parmentier which is a potato and leek puree garnished with minced chervil and croutons. As the onion flavor is mild it goes very well raw in salads too or as a last minute addition to stir fries.