Mon

In the past warriors used designs on their banners and camp equipment to identify who they were, these are called mon. It eventually spread to clothing. They often can be found on formal Kimono at the front just below the shoulder, between the shoulder blades, and on the sleeves. Crests can also be found on lanterns in front of houses and stores to make them identifiable in the dark. Mon continue to play an important role in Japanese society today. Patterns based on centuries-old family crests (called kamon) are often used as company symbols, trademarks, or as central themes for posters.

See this wikipedia article for more details and some history on mon: Wikipedia: Mon(crest). The wikipedia article also links to more articles on mon.

The above picture is taken from the Ranma manga, it is of Happosai memory of when he was 18. In the picture on Happosai (the guy in the picture), a mon can be seen.