The Ardent Epicure

The lentil is a legume that produces seeds.  These seeds are consumed in many areas of the world, particularly in the Middle East and Asia.  In some countries, the production of lentils is akin to the production of rice in many other countries.  They are produced and consumed in mass quantities all over, loved for their rich taste and high nutritional value.
Lentils are used in many dishes, and as a side, very similar to beans.  They are often used in soups, as they lend a great texture, some thickening, and a wonderful infusion of protein and vitamins, while also making the soup filling.  They are also used in salads, and many other dishes.
Types of Lentils
Lentils are all small and somewhat round-shaped.  They have a very distinctive shape that sets them apart from other products, round but flat, similar in shape to a lens.  They come in many colors, ranging from green, to reddish-orange, to brown, to yellow.  They are often mixed, and can provide a burst of color on their own, or in many dishes. 
While there are some flavor differences (as well as differences in cooking) between the different types of lentils, for the most part they are similar, and may be used interchangeably, or mixed together.
Choosing Lentils
Lentils are often sold pre-packaged, or may be purchased in bulk from large bins.  Look for lentils that are in good condition, and are not moist.  Be sure that the containers (if bought in bulk) look clean, and that their are no signs of insects.  When you buy bulk lentils, be sure to wash them before use, to remove any dirt or rocks that may have been left behind.
One of the best things about lentils is the option of color.  Lentils come in a variety of bright colors, and may be mixed well to provide a slight contrast in texture, as well as an array of attractive coloring for dishes.  Look for red lentils or black (beluga) lentils to add wonderful color contrast.
Storing Lentils
You should keep your lentils in an air-tight container, similar to what you would use for rice or beans.  Keep them in a cook, dark place, such as a cabinet, out of the sun, and away from moisture. 
Once your lentils are cooked, place them in a container with a lid (such as a plastic storage container), or in a bowl covered with plastic wrap, and keep them in the refrigerator.  They should keep well for several days.
Benefit of Lentils
Beyond being colorful and tasty, lentils boast much more on their résumé.  They are inexpensive, and thus are often used (along with rice and beans) in many poorer countries, to provide a sense of fullness, along with important nutritional value, to those who cannot afford meats and other nutritious foods.  Lentils are high in protein, dietary fiber, vitamins and minerals, and are a great source of iron.  How many vegetables can say that? (Well, I suppose they can't actually say anything, but if they could...) Since this is the case, lentils are a very popular source of protein, iron and vitamins for vegetarians.
Lentils are also very easy to cook.  Unlike normal beans, they do not need to be soaked before use, and they cook rather quickly, making them a great choice for fast, simple but nutritious dishes. 
Easy, cheap, tasty, and nutritious?  What's not to like?
Preparing Lentils
As mentioned, lentils should be washed and checked for dirt or rocks before use. Also be sure that none of your lentils appear to have been damaged by insects, and are in good condition.  Lentils  may be cooked at a low boil, and will take up to about an hour to cook.  Depending on your lentils, they may take less time, so you should check them often while cooking.
Tip: lentils take on spices very well.  Season your lentils liberally, and you will bring out the natural, earthy flavor of the lentils, and lend great flavor to any dish.  Also note that, unlike pasta, salt should not be added to the lentils while you are cooking them (this tends to make them touch); add salt once they are finished cooking.
Serve lentils by themselves, with rice or other grains, or with another dish, such as fish, lamb, or poultry.
For more information on the specific nutritional benefits of lentils, check out The World's Healthiest Foods. 
For a great recipe for a tasty lentil salad, check out Alton Brown's Recipe on the Food Network.