Currency war, also known as competitive devaluation, is a condition in international affairs where countries compete against each other to achieve a relatively low exchange rate for their own currency. As the price to buy a particular currency falls so too does the real price of exports from the country. Imports become more expensive. So domestic industry, and thus employment, receives a boost in demand from both domestic and foreign markets. However, the price increase for imports can harm citizens' purchasing power. The policy can also trigger retaliatory action by other countries which in turn can lead to a general decline in international trade, harming all countries.

http://en.wikipedia.org/wiki/Currency_war

Worth looking up the Yen. Most of the macroeconomists have been arguing that the first volley was fired by Japan early last year.