There are lots of names and designations for China's currency. Renminbi, Yuan, Kuai or RMB, CNY and ¥. Please find below a short overview of what means what and other details to money in China.
Renminbi (Chinese: 人民币) literally means people's currency. It is China's official currency and is abbreviated with RMB. The Yuan (Chinese: 元), also used as the name for the the country's currency, is the basic unit. The Yuan's sign is ¥ which is also used for the Japanese Yen. One Yuan or RMB equals 10 Jiao (Chinese: 角) or Mao (Chinese: 毛) which in turn is subdivided into 10 Fen (Chinese: 分). While you might receive a Jiao or 5 as change, Fen is not used anymore in everyday life. The International Standardization Organization uses CNY as their abbreviation for Chinese Yuan. The word Kuai (Chinese: 块) actually means piece and is often used in spoken Chinese. It's the colloquial designation of the Yuan but is also used sometimes for the Hong Kong and US Dollar.
Renminbi bank notes are available in denominations of 100, 50, 20, 10, 5 and 1 Yuan as well as 5 (0.5 ¥) and 1 (0.1 ¥) Jiao. Coins are available in denominations from 1 Jiao to 1 Yuan (0.1¥, 0.5¥, 1¥).
While ATMs and banks like Bank of China, China Construction Bank, Agricultural Bank of China or the Bank of Communications are very present in every bigger and midsized city it might be hard to get cash in rural areas. Please note that usully all ATMs accept foreign credit and banking cards but this depends more on the individual ATM's mood than on any regulation or logic. If one doesn't work, just go to the next one.
Please don't expect that paying in US Dollars, Euro or any other foreign currency will be accepted. It is recommended to have a fair amount of cash RMB on hand.
The acceptance of foreign credit cards is rather low in China. While they can be used in large hotels and international stores, restaurants for instance usually only accept Chinese cards or cash.