MetaTrader 4 is a platform for trading Forex, analyzing financial markets and using Expert Advisors.
Forex can affect the lives of everyone, regardless if you don't travel overseas or don't invest in currency. Today's world of commerce is such an international one that happenings on the other side of the world can ripple out to all nations.
China is a perfect example of this. The government in China regulates the exchange rate of their currency, and many believe the currency to be undervalued. A undervalued currency means Chinese made goods can be purchased for "less" on the international market. Were the Chinese government to allow the market to dictate the exchange rate the effects would definitely be felt across the globe, even for Americans who've never left America.
The reasons why an individual - or institution - would want to exchange money range to a myriad of different reasons, but the 3 main demographics include large corporations and institutions, speculators (investors) and tourists.