Udon are thick white noodles made with flour, salt and water. It can be cooked hot in Katsuobushi (dried bonito) or Kombu (dried kelp) broth, or prepared cold with Mentsuyu dipping sauce (Zaru Udon). Udon itself has a very mild flavor and can be seasoned with any kind of flavor you like. It can be used not only in broth but also with stronger flavors like curry soup. It is also very good good stir-fried (Yaki Udon).
The most common kinds of Udon found in stores in the US are dried and frozen. Dried Udon is much thinner and when cooked is usually pretty soft. It is a convenient choice because it has a long shelf life in the pantry. You need to be careful not to overcook dried Udon especially when you prepare in hot soup. Dried Udon may not be suitable for stir-fry because the thin and soft noodles may stick to the pan too much. The frozen kind is firmer and has a chewier texture, and it’s more forgiving for overcooking; however, it may be harder to find in local stores, other than Japanese stores. You could also make fresh noodles at home. Homemade fresh noodles are elastic and even firmer than frozen udon. It can be cooked in any ways, but it may be best cold to enjoy the texture.