Eastern Asia is the second most populated location on planet earth (Southern Asia is the first), and includes Mongolia, Japan, North and South Korea, and China. There are over one and a half billion people living in Eastern Asia, and its major religions are Taoism, Christianity (mostly in South Korea), Shinto, Shamanism, Confucianism, and Buddhism.
The largest country in Eastern Asia is China, also known as the People's Republic of China. There are 1.3 billion people living in China, which makes it the most populated country in the world. China is the third largest country in terms of total area, and with 2.3 million troops they have the biggest military on earth. China is considered by many to be a superpower, and is controlled politically by the Communist Party of China (or the CPC). The capital of China is Beijing, the second largest city in the country (the first is Shanghai). Although the Chinese government is an authoritarian government which often restricts the civil liberties of its own citizens, it's still a generally safe and fun place to visit as a tourist. In fact, China is the fourth most popular tourist spot in the world (predicted to be number one in the year 2020 by the World Trade Organization), pulling in around 200 billion dollars per year.
The administrative region of China - Hong Kong - has seven million citizens. While Hong Kong belongs to China, they are for the most part left to their own devices and are allowed to be a capitalist region, at least for the next forty years.
Tibet, the highest region in the world, is a disputed area. The Communist Party of China thinks it owns Tibet. The Government of Tibet in Exile disagrees, but doesn't have the power to do anything about it. While it is currently being taken over by Han Chinese, the indigenous Tibetan people are a peaceful and spiritual sort who practice Buddhism and follow Dalai Lamas.
Taiwan is an island governed by the democratic Republic of China (or ROC), which is different from the People's Republic of China, although the PRC would like very much to remedy that. Life in Taiwan is dominated by the fact that the PRC could decide to force a merger at any moment.
Mongolia is a parliamentary republic that sits right next to Russia. Mongolia is an independent country with a constitution providing civil liberties to its citizens, and maintains a good relationship with the PRC.
Japan is an island in the Pacific Ocean. The Land of the Rising Sun is a constitutional monarchy, which means it's run in the same fashion as England, only instead of the Queen there's an Emperor. Japan is close friends with the United States, and receives over eight million tourists per year.
In between China and Japan sits North and South Korea. South Korea is a moderately popular tourist location (mostly for Asian tourists), attracting six million tourists per year.