Here's a photo of the four types you're most likely to see. They are described, left to right, below the picture.
This is the traditional lentil that is most commonly used for soup. To be honest, I never use this type of lentil. Advantages: they're cheap, readily available and cooks quickly. Disadvantages: when you overcook them (easy to do) they fall apart and get mushy, and to me the flavor is muddy and not very pleasant.
So, with that endorsement, these are best used in lentil soup where the mushiness can dissolve into the soup. I brighten the flavor of a green lentil soup with lemon juice, sherry vinegar or balsamic vinegar.
Lentilles du Puy
These are a French lentil. They have a deep, rich flavor and have the advantage of not falling apart when you cook them. As a result, they can be used in soups and warm or cold salads. They are excellent in the Lentil Soup recipe on this site.
You can make a really warming dinner of warm cooked lentilles du Puy tossed with a mustardy vinaigrette, sauteed onions and black pepper. Serve this alongside browned kielbasa or other ham or sausage. Another nice thing to do is toss the cooled lentils with diced beets, crumbled goat cheese and a sherry vinegar and walnut oil vinaigrette. Serve over lettuces with some toasted rustic bread (walnut bread would be nice).
David Lebovitz has a really nice discussion of these lentils (and a good method for cooking them for salad) on his web site at this link: http://www.davidlebovitz.com/archives/2006/11/cheap_caviar_1.html
Black "Beluga" Lentils
These lentils are little smaller than lentilles du Puy. Cook them in the same way though. They are best in salads and they are so pretty on the plate. They stay very dark in color and look beautiful against roasted yellow peppers, white goat cheese, purplish radicchio and rich greens.
These are for soups or purees only. Red lentils fall apart when cooked. This is a great advantage when you know what's going to happen. If the first time you met these and you were expecting them to behave like a green lentil, you may have learned this lesson the hard way.
I will be posting an Indian-style soup recipe for these later this week. They are also easily cooked for a side dish: For four people, use 1 cup of red lentils. Put them in a saucepan and cover them by 1/2" with chicken broth or water or both. Toss in a crushed clove of garlic, a pinch of cayenne, and salt and pepper. Bring to a simmer, stirring occasionally. When they are done, the lentils will have turned into a soft orange puree. (To keep the color bright, add a pinch of turmeric when you start the cooking.) Season to taste with salt and pepper (white pepper is nice) and a squirt of lemon or lime juice.
What is a lentil? http://en.wikipedia.org/wiki/Lentil