Urad beans are a small black-skinned bean that is pale yellow on the inside and have a slightly glutinous texture. The beans can be prepared and eaten as a side dish or they can be pureed and added to soups.
In India, dumplings (idlee) are made with the fermented beans as well as pancakes called dosai.
You can purchase the beans in Indian markets. They are available whole, split, skinless or with skins on. You can also purchase ground urad flour.
If you don't have urad, you can use equal amounts of one of these substitutes: