Kombu is a variety of bull kelp. In Japan a huge range of seaweeds (kaiso in Japanese) are harvested for food purposes. You may be familiar with wakame, iwa-nori, dried nori etc. In Japan kombu is often eaten "as is", but is usually used as a "base"; to create a stock, much as in the West we use chicken or beef to create stocks. Kombu-kelp contains glutamic acid, which enhances flavour and gives the body to kombu stock.
Kombu has a "double layer". Much like an envelope, the two outer layers can be opened and a space like a package within can be created for storage and cooking. When harvesting kombu, the root of the kombu is not touched; rather the top "leaf" portion is taken, so that from the base (or root) of the kombu can regrow. It really is the ultimate renewable food resource. There is a food called ne-kombu which does utilize the root, and while nutritious and delicious to eat, we do not supply as it effectively kills the plant.
In Japan there are many levels of Kombu-kelp, and this is mainly decided by the age of the kombu. First year kombu is quite thin (and so is the taste), compared to second year ma-kombu. Kombu growth drops off after it's first year, and from the stump, a new and superior second year kombu-kelp grows.
Most of the kombu-kelp in the market is first year kombu, and the difference is obvious when you make the dashi-soup with it. Matsumaeya uses only second year kombu (called ma-kombu in Japanese, "ma" meaning "true" or "real"). While more expensive than the readily available first year kombu, the difference is immediately obvious. Ma-kombu has a depth of flavour unlike any other, and is the only choice in Japanese restaurants that aim for the highest quality.
Kombu is high in a wide range of trace minerals and amino acids, including potassium, iodine, calcium, and vitamins A and C, as well as B-complex vitamins and glutamic acid. Kombu can be safely stored for several years, as long as it is kept away from moisture and light.