Soup is the life blood

There are 3 basic types of ramen soup, with most places serving a blend
Tonkotsu based -
Made from pork bones (tonkotsu), this soup is heavier and greeasier than the others.  Tonkotsu ramen is most commonly served in Kyushu.  The noodles are generally thicker than those served with other flavors.  Shops that serve tonkotsu ramen generally serve kakuni as a topping.  The smell of the soup is generally cut with onions.
Chicken based -
Made from chicken bones, whole chicken, and vegatables, this type of soup is generally lighter and less greasy.  Depending upon the tare (flavoring) used, the noodles vary from thin to medium thickness.
Seafood based -
The main ingredient is usually bonito flakes with various shellfish (dried shrimp and scallops, mussels) added.  To cut the fishy smell sake (rice wine) and lots of vegatables are used.  The ligtest of the 3 this is generally only served in Shio (salt)
or Shoyu (soy sauce) flavors.