Udon is often served hot as a noodle soup in its simplest form, as kake udon, in a mildly flavoured broth called kakejiru, which is made of dashi, soy sauce, and mirin. It is usually topped with thinly chopped scallions. Other common toppings include tempura, often prawn or kakiage (a type of mixed tempura fritter), or aburaage, a type of deep-fried tofu pockets seasoned with sugar, mirin, and soy sauce. A thin slice of kamaboko, a halfmoon-shaped fish cake, is often added. Shichimi can be added to taste.
The flavor of broth and topping vary from region to region. Usually, dark brown broth, made from dark soy sauce (koikuchi shōyu), is used in eastern Japan, and light brown broth, made from light soy sauce (usukuchi shōyu), is used in western Japan. This is even noticeable in packaged instant noodles, which are often sold in two different versions for east and west.
(source: Wikipedia – https://en.wikipedia.org/wiki/Udon)