There is a currency war going on between most of the world Central Banks. A war without guns, but with currencies instead.
First a strong currency is good for companies importing goods and commodities and good for everyone who wants to travel abroad. A rising currency can also attract more foreign capital inflows into the economy.
A weak currency, is good for exporting companies and for tourists who wants to visit the country. Foreign goods become more expensive and domestic goods, therefore, become more competitive.
China may have started the present currency war we are seeing right now by tying the Yuan to the dollar, making it impossible for the Yuan to strengthen compared to the dollar even though rising economics usually brings a stronger currency.
This enabled the Chinese to sell cheap goods to the US while at the same time lending huge amounts of that trade surplus back to the US, who happily borrowed it to consume even more.
This worked fine until 2007 when the housing bubble burst (but you have heard that story before).
To soften the blow of the housing bubble the FED decided to slash interest rates to 0 %. When that didn't help the started QE, QE2 and later QE3 pumping enormous amounts of money into the financial system in the US.
This also caused the dollar to fall compared to all other currencies except the Yuan. So all of a sudden the worlds largest market became much harder to export to for all companies with costs in Euro, Yen, Canadian dollars etc.
At the same time the crash hit the whole world not only US, forcing almost all Central banks to cut rates dramatically.
Japan who has seen stagnant growth for over 20 years started an ambitious program called Abenomics what consisted of fiscal stimulus, monetary easing and structural reforms.
Shinzo Abe started a stimulus program of 10.3 Trillion Yen to get the inflation up tot 2 % and the Bank of Japan started its own rounds of QE of 60-70 trillion Yen each year, which was increased in 2014 to 80 trillion yen (660 billion dollars) per year.
Europe was one of the continents who may have gotten the hardest hit, Spain and Greece still have unemployment of over 25%. The ECB has announced that they are going to start their own rounds of QE by buying bonds for 60 billion euros a month (64.5 billion dollars). Together with a historic low for interest rates, -0.1%.
China is injecting 500 billion yuan ($81 billion) into the nation's largest banks on top of the extremely generous stimulus package they released in 2008. 4 trillion yuan (586 billion dollars) in infrastructure and social welfare was injected to keep the growth rates up.
Bank of England is sitting on £375bn (565 billion dollars) that they are ready to release into the economy when the interest rates rise from their historic lows.
This is just a short summary of the actions of the worlds largest central banks, many central banks are using their own QE to fuel the economy and sink their own currency.
The version they are telling to media is of course to boost their own economy, but the real motives are to lower their currency to make capital stay in the country and to make it cheaper for exporting companies to export goods to other countries.
So here we are stuck in this cycle of ever more capital into the system, because this tactic work great when you are the only one doing it, but since the whole world is on a quest of lowering their own currencies the cycle need more and more capital to gain effect.
Who is going to pay for all of this debt?