"Udon is noodles made from wheat flour, salt, and water. Usually, udon noodle is thicker and firmer than ramen or soba and whitish in colar. The meccas of Udon are Sanuki and Oosaka, the western parst of Japan. Most of udons found in Tokyo, imported from western parts are served in hot broth made of soy sauce, mirin (sweet sake), sugar, and dried bonito stock, but the styles varies by areas where it is served."

From:
http://www.worldramen.net/Varietion/udon/udons.html