Soba & Udon are two kinds of Japanese noodle. Soba (on the left in the photo above) is made from buckwheat flour and Udon (on the right) from wheat flour. They are served either in a soup or a sauce and are available either hot or cold in many delicious variations.
Soba noodles are a popular inexpensive fast food at railway stations throughout Japan and are also served by expensive specialty restaurants. There are a wide variety of dishes, both hot for winter and cold for summer, using these noodles.
Udon are thicker noodles than soba and are often served hot as a very simple soup made of dashi, soy sauce, and mirin. It is usually topped with thinly chopped green onions and comes in many regional variations. One of the most famous variations is Sanuki Udon from Kagawa Prefecture in Shikoku. It is noted for its square shape and flat edges.