Rokugan is the world of the game Legend of the Five Rings. It is a close analogue of various Asian countries, China, Korea, but mostly Japan. It is set during the fuedal period of that area, with emphasis on samurai warfare.
Rokugan is also the default world for the new Oriental Adventures book used by third edition Dungeons and Dragons, replacing Kara-Tur to a degree. (I'm quite sure that TSR will produce a separate Kara-Tur book, however.)
Rokugan is divided into "clans" which are all named after animals, both fantastical and real. (The card game uses these divisions to create a loyalty among the fans for individual factions... Most of the factions listed below have a card game faction alignment.) There are the major clans: The Unicorn Clan, The Dragon Clan, The Scorpion Clan, The Lion Clan, The Phoenix Clan, The Crane Clan, and The Crab Clan. Most recently, the Mantis Clan was added to that list. (And are in danger of losing that status, in the recent storyline.) There are also minor clans and other organizations: The Fox Clan, The Centipede Clan, The Badger Clan, The Monkey Clan, The Hare Clan, the the Brotherhood of Shinsei, The Living Shadow. There are a few non-human races that are involved, including the Naga and the Nezumi. And finally, there are the ultimate villains, The Shadowlands Horde.
Keeping with the tradition of xenophobia, Rokugan is isolated from the surrounding lands. True, their neighbors to the south (the Shadowlands) are a literal Hell on Earth, their neighbors to the west (The Burning Sands) and to the north (The Yabinjin) are more mundane. To the east is the ocean. (Which makes the larger Rokugan, in many ways, more like China than the island of Japan.)
Many things are incongruent with our real-world equivalent. Some were intentional (the designers wanted to make women, for instance, equals of men to encourage female gamers to play), but others weren't. (See my Legend of the Five Rings node to see a few examples.) To make the world a fantasy world, they made the preists (shugenja) able to cast magic based on their five elements (Fire, Water, Earth, Wind, and Void). Some argue that the five elements from the original Oriental Adventures are more "correct" (fire, water, wood, metal, earth), but L5R's elements are based on the the Book of Five Rings' elements.