There are a lot of different lentils, each suitable for different dishes. Lentils are cheap, healthy (loads of proteins, minerals/vitamins and dietary fibre), filling and versatile. Lentils do not taste like much themselves, but are excellent at absorbing whatever seasonings and spices mixed with them. See this page for more lentil types and pictures.
Often one of the cheapest type, this khaki-coloured type of lentils is suitable both in soup and salads. The outer layer tends to be hard and "chewy" while the inner parts are mealy. Boil them for 30-45 min. (they should be al dente and not too mealy), drain and dress them in olive oil/soya sauce or other spices while they are still steaming, this way, they soak of the flavour easily. Make a salad with fresh vegetables, use them in meat dishes as filler (seasoning isn't necessary in that case) or as a carbohydrate to your protein main course.
Red lentils (typically orange or salmon coloured) do not retain their shape when they're boiled and is therefore mostly suitable for soup or other "wet" dishes. It is very good in the Indian dish dal and other soups where the red lentils provides a thick, stew-like consistency. Red lentils turn yellow when they are boiled. They are a good choice for lentils newbies because the flavour and consistency of the lentils are masked.
Beluga lentils are excellent in salads: They do not have the same hard and chewy other shell as green lentils and are less mealy. Beluga lentils retain their round shape when boiled. They are named after the much more expensive beluga caviar thanks to their similar appearance.