Micro-USB as a standard charger
Nowadays, (nearly) all consumer devices can be carged using a USB cable, often using a micro-USB cable.
This is no coincidence. Both the Open Mobile Terminal Platform as well as the European Union's common External Power Supply specify USB with micro-USB cables. (Apple is the only company greedy enough to pick a different standard than micro-USB, but is selling a €19(!) convertor just to comply with EU regulations).
Douglas Adams would be proud of this level of standardisation.
The following power is available per USB port:
|USB 1.1||500 mA (5 unit loads of 100 mA each at 5 V = 2.5 Watt)|
|USB 2.0||500 mA (5 unit loads of 100 mA each at 5 V = 2.5 Watt)|
|USB 3.0||900 mA (6 unit loads of 150 mA each at 5 V = 4.5 Watt)|
|USB Dedicated charge port with standard USB type A connector||1500 mA|
|USB Dedicated charge port with special connector||5000 mA|
|Apple high power USB 2.0 or 3.0 (on Apple computers manufactured after 2007)||1100 mA (listed in system profiler as 500 mA available power + 500 mA extra operating power)|
For USB 1 to USB 3, a suspended USB port (with nothing connected) provides at most 2.5 mA power. As soon as a proper USB device is connected, it will default to a "low power" state able to supply 100 mA (USB 1 and 2) or 150 mA (USB 3). After negotiation, it can ramp that up to 500 mA (USB 1 and 2) or 900 mA (USB 3).
Apple uses a proprietory protocol that allows devices to negotiate up to to 1100 mA on both USB 2 and USB 3.
The above regular USB ports are called Standard downstream ports (SDP). The USB Battery Charging specification added Charging downstream port (CDP) (power + data) and Dedicated charging port (DCP) (power only), both of which allow higher power capacity. Usually up to 1500 mA (1.5 A) with regular cables and connectors, but with proper cables and connecters even up to 5000 mA (5 A). Another mayor differences with regular USB ports is that it is no longer needed to first negotiate the high power requirements. It is available right away. The advantage is that these ports can thus be placed in wall outlets without the need to implement the USB data protocol.
Detailed information can be found at The Basics of USB Battery Charging: A Survival Guide (Maxim tutorial 4803) by Len Sherman.
Most devices are perfectly able to charge with the 500 mA provided by USB 2.0. That said, more power is usually better, since it allows devices to charge faster.
Apple devices (iPhone, iPad) will usually benefit when plugged into to an Apple device because of the "high power" USB features.
iPad Charging Requirements
The iPad has hefty power requirements, and will display the message "Not Charging" when plugged into to power sources that privide less then 1200 mA, such as regular USB 2.0 and even most USB 3.0 ports. This is not true: the iPad is charging, just very slowly.
It is recommended to charge an iPad using a iPad charger, if only for speed. The iPad 1 and 2 have a 25 Ah (Ampère-hour) battery, and the iPad 3 and 4 have a 42.5 Ah battery. Charging an iPad 3 or 4 using a USB 3 port at 900 mA or a Apple higher power USB port at 1100 mA thus takes at least 39 to 47 hours. Using the included 12 Watt (2.4 A at 5 V) charger reduces this to 18 hours. (Note: I've seen charge times of about 6 hours. If you can explain me the difference, please let me know!)