Although ramen is very popular among young Japanese and considered as a national food of Japan, ramen originally came from China. It is not exactly the same but Japanese adapted the Chinese noodle to Japanese ramen.
On the other hand, udon noodle is the another main noodle in Japan. The ingredients are simple. Wheat, water and salt. Then, ramen uses animal based stock as like pork or chicken, but udon uses fish or seaweed based stock. Which means udon is less heavy than ramen.
Then, ramen always has various toppings(stewed pork, bamboo shoot, bean sprout, boiled egg and etc.) on it, but udon is rare to mix the toppings. Tempura udon just comes with tempura on it and kitsune(fox) udon comes only with fried tofu.
May be the most famous udon is Sanuki udon provided in Kagawa prefecture, former Sanuki province. The noodle itself is relatively harder than other udons. Then, usually it is served as self-service which means you have to order at the counter and take it with you to your table by yourselp as like the cafeteria. It is common that you can choose the side orders on the spot. Side orders can be various types of tempura(prawn, cuttlefish, chicken, vegetable and etc.) or rice balls.
Then, the noodle can be warm or cold. I have chosen cold udon and side orders were ika-ten(cuttlefish tempura), chikuwa-ten(fish cake tempura) and mixed tempura - kakiage.