Red lentils are a type of shelled lentils. They're quick-cooking, and will keep their shape if cooked for 5-7 minutes, will be mushy if cooked at 15 minutes and will turn to soft puree at 25 minutes.
Lentils, or Lens culinaris, is a legume cultivated for its lens-shaped seeds, and they've got the second-highest ratio of protein per calorie of any legume after soybeans. Lentils are also a rich source of zinc and iron.
Lentils can be soaked, germinated, boiled, fried and baked. They require a cooking time of 10-40 minutes, depending on variety. The cooking time is shorter for varieties with the husk removed, like red lentils. Lentils without husks tend to soften into purees, while the husked varieties remain whole. When cooking lentils, add salt after cooking, not during, as salt will keep them tough.
Lentils contain antrinutrients, which reduce the bioavailability of dietary minerals. To improve bioavailability, lentils can be soaked, fermented or sprouted (ref).