Sumo is Japanese traditional wrestling where wrestlers try to force their opponents out of small circular ring or make opponents touch the ground with any part of their body. Sumo is not just a sport to Japanese people since it has ritual and religious background. It is usually said that sumo is somehow related to Shinto, which is Japanese traditional religion.
Sumo has some aspects which cannot be understood well in modern society; women cannot even enter the ring. It is famous that sumo was criticized by people all over the world recently due to the fact that they barred one woman who was giving first aid to a man who collapsed in a sumo ring. They say women cannot enter here because sumo ring is considered to be sacred. Well… I cannot agree, though.