The Dal (split and peeled) form of Masoor is highly valued in fuel short India, for its short cooking time and flavor. It is often cooked with rice, as the cooking time is similar. The Red Lentil is very similar, but more characteristic of North America.
The photo shows Masoor Dal to the left, about 0.17 inch (4.3 mm) diameter, with whole Msoor in the center, about 0.22 inch (5.6 mm). To the right are North American type Red Lentil Dal, about 0.24 inch (6.1 mm) diameter, however, some imported Red Lentils are as small as the Masoor Dal.
The peeled and split versions of these lentils are sometimes called "Egyptian Lentils" which an inexact term, usually meaning Red Lentils, but is also sometimes applied Masoor, and even Brown Lentils.
More on Varieties of Bean,
Peas and Lentils.
Buying: Whole Masoor and Masoor Dal are easily available in markets serving an Indian community, and Red Lentils are widely available in North American supermarkets.
Storing: Keep lentils or dal cool and dry in a sealed container. Mark the container by date purchased, because age affects cooking properties. Technically they'll last for years without spoiling - BUT in practice they should be used within a year, as vitamin content declines and cooking time may increase, though I have found 2 year old Masoor Dal still cooked well.
Soaking: Whole Masoor can be cooked without soaking, but will cook more quickly if soaked for 4 hours or more. Soaked, it will also be more tender and fewer will break open. For unsoaked it will take about 35 minutes. For 1 cup use 2-1/2 cups water and 1/2 teaspoon of salt. Yes salt - see our page on Soaking / Brining Dried Beans.