There are several kinds of lentils. The most common are pink or red; these form a puree as soon as they are cooked. If whole lentils are required, the green or brown variety must be used. The best (and most expensive) lentils are the black lentilles de puy which come from the Auvergne in France. Lentils are excellent served cold in salads.
Lentils do not need long soaking like peas and beans, but they can profitably be soaked for 1-2 hours before cooking.
Put the lentils into a pan and cover with cold water or stock. A small skinned onion, a few parsley stalks, or a bouquet garni, a ham bone, or a few bacon rinds can be added to give extra flavour. Bring the water to the boil and simmer gently for about 1 hour or until the lentils are quite tender. Drain and remove any herbs and flavouring. Re-season if required, and serve hot. If using the pink lentils, drain and puree them before serving.