There are three basic types of lentil available in the UK. These are:
Whole Green Lentils: A really versatile lentil, and the very best sort for sprouting. Breaks down in about an hour with long, slow cooking, making it ideal for wet dahls where you want a lot of sauce.
Whole Brown Lentils: Similar to green, but not as good at sprouting. When peeled and split, they're sold as Red Split Lentils, often used in soups or for very fast dahls and curries.
Lentilles Vertes: The main difference between these and Puy Lentils is the price. Puy lentils are grown in the French prefecture of Le Puy, cost considerably more, and are more likely to harbour small stones; otherwise they are the same. Lentilles Vertes have a subtler taste and firmer texture than the humble green or brown varieties. They are great for dry dahls (those with less sauce and more bite).