Dal, Dhal, Dahl
Let me first say that I get annoyed by all the different spellings of Dal. Stick to Dal. It is the most commonly used spelling, and avoids you trying to transliterate from any of the dozens of Indian languages. English does not have the "dh" sound anyway, and I have never heard a non-Indian person pronounce it.
Dal can mean one of two things.
1) a lentil, any lentil. Often put after the name of the lentil eg urad dal, red dal, toor dal, chana dal.
2) A dish made from the dal (lentil) which consists primarily of the dal (lentil) and spices, with some optional vegetables. It generally has a "wet" consistency, but not soup-like, and unlikely to be the thick mass that is found on many non-Indian recipe sites.
Dals (lentils) are high in proteins. They are an integral part of an Indian' vegetarian diet.
Each dal generally can be purchased in 5 different forms.
1) skin on, whole lentil
2) skin on, split lentil
3) skin off, whole lentil
4) skin off, split lentil
5) as a lentil flour
Each type has a very different flavour. Mung beans for example. Skin on types have earthy flavours, whereas without the skin they are quite sweet.
A rule of thumb is that dals with the skin on need to be soaked overnight, while the ones with the skin off often don't need to be soaked or for a much shorter time. There are exceptions (eg urad dal).
Different regions of India have local preferences for most-used dals, as to be expected.