I will offer you the most concise answer possible to such a broad question. Basically, “Ancient China” is referring to the dynastic ages (around 2,000 years) leading up the last emperor of the Qing Dynasty in the early 1900s. After that, there was a civil war between the Communist Party and the Nationalists. The Communists won, and in 1949 they established the People’s Republic of China (PRC) which henceforth was known as “New China.” However, party chairman Mao Zedong’s rule for the next 30 years was very brutal, during which tens of millions of people were executed by the Communists. Not until after Mao died in the last 1970s did China open up its economy to the modern world; so, from the 1980-present, we call this “Modern China.” Again, I have put it very, very simply. It’s much more complex than this.