The currency market is the largest market in the world with trillions of dollars worth of global currencies traded. This market is larger than equities, oil and bond markets. One of the reasons that currency markets are bigger than equity markets is because central bank are the major participants in the market. On a daily basis these banks and other institutions trade currencies with one another.
However currency trading is not suitable for most retailers for many reasons. For example, currencies are extremely volatile and can move either direction for any number of reasons including political stability or the lack of it. Hence the majority of retail investors are better off to stay out of the currency markets.
The chart from Callan Associates below shows the annual returns of major currencies from 2006 to 2015:
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Source: Callan Associates