元 [yuán] is the currency of China. It is written on the coins and notes (as 元). 元 [yuán] es la moneda de China y está escrito en las monedas y billetes (como 元).
块 [kuài] is used when speaking informally (e.g. in shops, etc). 块 [kuài] means "piece", "lump", "chunk", and it is used as a classifier for items such as piece of cake, for counting watches,..., and, of course, it became a colloquial classifier for money and monetary units.