What is the currency in China?

There is only one currency inside the mainland. RMB, REMINBI, YUAN, KUAI; these are all basically the same thing.  ¥ or 元

When written in English they tend to use RMB

Reminbi is formal, like you would never say, “I have 100 dollars of united states currency”, so not used in spoken language.

Kuai is like slang and it actually spoken for smaller amounts of money

Yuan would be used the most when speaking.  To read “Yuan” it sounds close to “You” + “N”

Taiwan, Hong Kong and Macau have different currencies, but there is only one currency used in the mainland.

See the following post for information on credit cards/cash/Wechat/Alipay: Can I use my credit card in China? Cash? Alipay? Wechat?