The simple answer is MONEY.
Because you're not buying anything physical, this kind of trading can be confusing.
Think of buying a currency as buying a share in a particular country, kinda like buying stocks of a company. The price of the currency is a direct reflection of what the market thinks about the current and future health of the Japanese economy.
If you've ever traveled to another country, you usually had to find a currency exchange booth at the airport, and then exchange the money you have in your wallet (if you're a dude) or purse (if you're a lady) or man purse (if you're a metrosexual) into the currency of the country you are visiting.
You go up to the counter and notice a screen displaying different exchange rates for different currencies. You find "Japanese yen" and think to yourself, "WOW! My one dollar is worth 100 yen?! And I have ten dollars! I'm going to be rich!!!" (This excitement is quickly killed when you stop by a shop in the airport afterwards to buy a can of soda and, all of a sudden, half your money is gone.)
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