The US is winding down its third round of quantitative easing (QE3), having been joined by QEs in the UK and Japan (Abenomics) in recent years.  The purpose of these programs is always to provide effective monetary policy when interest rates hit the zero lower bound. At zero the central bank cannot simply lower discount rates further. Existing versions of QE all rely on central bank purchases of (mostly) treasury bills which increase the demand for these assets, raise their price and thus lower their interest rate. By depressing interest rates on secure assets the central bank hopes to entice investors to loan out their money to riskier but higher yielding investments – such as business investment. 

The European version of QE started June 5, 2014 but it took a different form than outright goverment bond purchases. In the EU its a bit tricky to purchase goverment bonds, since there are so many goverments with different levels of debt! So the Europeans started with a new version of QE: paying negative interest rates on the deposits that banks have at the central bank. Just like citizens deposit funds at commercial banks, commercial banks at times deposit extra funds with the central bank. Now the European Central Bank will be charging commercial banks to do so. In effect the hope is that commercial banks move their money from the ECB and provide it to business men and women to stimulate the economy. We will see how effective this measure will be. 

Clearly the US, UK, and Japan could have gone similar routes. Instead especially in the US the central bank decided to pay interest on commercial bank deposits (albeit only 0.25%). Some suggest that the FED's reluctance to charge negative interest rates is related to the worry how it would affect commercial banks' balance sheets.