Hamon (from Japanese, literally "blade pattern") is a visual effect created on the blade by the hardening process. The hamon is the outline of the hardened zone (yakiba) which contains the cutting edge (ha). Blades made in this manner are known as differentially hardened. This difference in hardeness results from clay being applied on the blade prior the cooling process (quenching).
Have you ever seen real samurai swords? They are reallly really cool and beautiful! Katana or nihonto (Japanese name of samurai sword), is known throughout the world for its beauty and sharpness. You can see the artistically decorated scabbard, and also the wave pattern of the blade called hamon, which are the characteristics of katana.