The simple answer is the currency market. The market is made up of banks, governments, companies, tourists, anyone (you and I) that exchanges currency. It is simple supply and demand. When demand for one currency against another increases without an increase in supply the market value for that currency increases. When supply increases (ie The Central Bank prints more money) without an increase in demand the market value of that currency paired against other currencies decreases.
There are many more factors that influence the demand or supply of a currency but this is the basic concept.

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