Leeks are a close first cousin to the onion. They have a milder flavor. They are native to Europe, and can be braised, served in various sauces, au gratin or used in soups and sauces. A smaller variety is called Welsh Leeks, however, these come from an area in Siberia.

Leeks look like giant green onions which makes good sense as the leek is related to both garlic and the onion even though its flavor and fragrance are milder and more subtle. Because they're so sweet, leeks are often cooked and served as a side vegetable. 

Select leeks with clean white bottoms and crisp, fresh-looking
green tops. Small to medium leeks (less than 11/2 inches in
diameter) are the most tender.  

Refrigerate, unwashed, in a plastic bag for up to 1 week. 

Remove any discolored outer leaves. Cut off and discard
the darkest green tops. Trim off the root tips. Cut the leek in half lengthwise, spread the leaves and rinse out the dirt. Bake, broil, braise, sauté or microwave. Serve alone, seasoned with butter or margarine and
herbs. Boil, drain and place in a casserole; season with grated
cheese and bread crumbs, then bake until bubbly. Use with egg dishes, like quiche. Add to stews, soups and stocks. Can be sliced thinly and added to salads raw. One of the main ingredients in French vichyssoise, a rich, creamy potato and leek soup that's served cold. 

Here are a few good leek sites to link into:
http://www.science.uva.nl/~mes/recipe/usenet/drunk-leeks.html

http://www.detnews.com/1998/food/9801/07/01060051.htm

and on another note, here is a very intriguing poem about the lovely leek
http://www.nhumanities.org/leeks.htm