"Amazing strides have been made in recent years towards standardized chargers and connectors for smartphones, tablets, MP3 players, and many other gadget species. In just five years — with the obvious exception of Apple’s iOS connector — Micro-USB has single-handedly destroyed the industry’s penchant for wildly customized and proprietary connectors. Today, you can charge your phone at your friend’s house, plug your Kindle into any computer, and download photos from a digital camera directly to your TV, all thanks to a standardized connector.
In its place, though, another problem has arisen: USB power. Not all USB chargers, connectors, and cables are born equal. You might’ve noticed that some wall chargers are stronger than others. Sometimes, one USB socket on a laptop is seemingly more powerful than the other. On some desktop PCs, even when they’re turned off, you can charge your smartphone via a USB socket."
http://www.extremetech.com/computing/115251-how-usb-charging-works-or-how-to-avoid-blowing-up-your-smartphone
