Background refresh is part of what allows any iOS 7 app for iPhone or iPad to appear to be multitasking all the time. The truth is, they're really multitasking "just in time" by updating in the background whenever they have the opportunity or right before they figure you'll be launching them. In spite of how efficient Apple has tried to make it, however, using background refresh will also use more power and consume more data. Luckily, if either your battery life or your data limit is running low you can turn off background refresh, either entirely or just for those apps you don't consider that important.
Full story from the iMore Blog...
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