Brown Lentils - This is by far the most common variety of lentil, and probably the one that you see at your local grocery store. They can range in color from khaki-brown to dark black, and generally have a mild earthy flavor. They cook in about 20-30 minutes and hold their shape very well. Common varieties are Spanish Brown, German Brown, or Indian Brown. The blackest and tiniest lentils you find are usually Beluga lentils, which have a rich and deeply earthy flavor.
Green Lentils - These can be pale or mottled green-brown in color with a glossy exterior. They have a robust, somewhat peppery flavor. Green lentils generally take the longest to cook, upwards of 45 minutes, but they keep a firm texture even after cooking. This makes them ideal for salads and other side dishes. Look for Lentilles du Puy, Puy lentils, or French Green lentils.
Red Lentils - With colors ranging from gold to orange to actual red, these are the sweetest and nuttiest of the lentils - to our mind, at least! They're somewhere in the middle in terms of cooking time and are usually done in about 30 minutes. They tend to get mushy when cooked through, so they're perfect for Indian dals and other curries, or for thickening soups. A few varieties are Red Chief and Crimson, and you'll often find them in Indian or Middle Eastern markets labeled as masoor (red lentils) or channa (yellow lentils).
Do you have a favorite kind of lentil? What do you like to make with them?