Sumo wrestling is the Japanese traditional national sport which was originally a Shinto ritual for good harvest about 1500 years ago.
Two sumo wrestlers fight on a 4.5 meter ring, called 'Dohyo' made of clay. A wrestler wins when he pushes his opponent out of Dohyo, or when his opponent's part of body touches the ground except his soles.
There are six tournaments held in a year, lasting for 15 days. (three are held in Tokyo, and one each is held in Osaka, Nagoya, and Fukuoka.) During the tournament, you can see many colorful flags which have wrestlers names around the stadium.
Many Japanese people like sumo, because in sumo, a smaller wrestler can sometimes beat a bigger wrester.
In Tokyo, there are many sumo stables where wrestlers live and train together under the coaching of stable masters. At the JR station, 'Ryogoku', you can see some display concerning famous sumo wrestlers.