is the pinyin Romanization
of the Chinese
name for the money of the PRC
-- literally, "people's money". It is technically a mass noun
and should only be used to refer to money in the aggregate.
However, both Chinese people (when not speaking Chinese) and Westerners commonly refer to "20 RMB" or "1500 RMB".
The correct name of the unit, corresponding to dollar or ruble, when pronounced in putonghua, is yuan
In ordinary use, Chinese people speak of kuai. This is actually the measure word for flat rectangular things. You will rarely hear anyone say Gei wo yi yuan (= "Give me one ...") but you will always hear Gei wo yi kuai.
Bankers and money changers usually use the term CNY.
The correct way to write the unit of currency is, of course, with a Chinese character:
See a more detailed discussion of Chinese currency.