Leeks have a very long history: a 4,000-year-old Babylonian tablet recommends leeks crushed in lamb stew, and ancient Egyptian carvings, drawings and even specimens of leeks survive from the second millennium BCE. 
They were grown widely in the Roman empire; the emperor Nero, who liked to give public recitals, claiming that they improved his singing voice. For more than a thousand years they have been the national emblem of Wales (as well as that of Oltenia in south-west Romania).  However, during the sixteenth and seventeenth centuries they disappeared almost entirely from the tables of the gentry, possibly eclipsed by arrivals from the new world, or possibly made unpalatable by overcooking, which was widely recommended as the only way to prevent them causing bad breath.
Allium ampeloprasum, var. porrum, a.k.a. allium porrum
During the First World War the gardens of Versailles were given over to the cultivation of 25 million leeks, which were fed to the fighting troops. More recently leeks have become the object of the Northumbrian passion for giant exhibition vegetables, with specimens reaching 20 inches in circumference.
Leeks were a traditional ingredient in many folk remedies; considered an anti-inflammatory, they were applied externally as a hot poultice, and are still regarded as effective against respiratory ailments, being highly mucilaginous and therefore soothing.
Leeks can be divided broadly into summer varieties and over-wintering ones, the latter being extremely hardy and having a stronger flavour. They can be harvested from finger-sized to much larger; lifting them with their roots on extends the time they can be preserved.  The edible part is the white onion base and the bundle of leaf sheaths, usually blanched from their method of growing, well submerged in earth. The darker part of the leaves is sometimes tied with twine and other herbs to make a bouquet garni. As a cooking vegetable they are extremely versatile and can be steamed, boiled, braised or fried, as well as being an ingredient in many soups and stews. Young leeks can be used in salads.