USB Power

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 Douglas Adams would be proud of this level of standardisation.

Side note: Apple is the only company that chose Lightning port over micro-USB, mostly because Lightning can stream media, while micro-USB can't. Unfortunately, Lightning is a proprietary format, thus other companies can't use it without paying hefty fees to Apple. Moreover Apple is greedy enough to charge €27 for a cable and €19 for a convertors.

Power Available
The following power is available per USB port:

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.

Power Requirements
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 Wh (Watt-hour) battery, and the iPad 3 and 4 have a 42.5 Wh battery. That's the amount of energy. Given the Li-Polymer power of 3.6 Volt, this comes down to a of capacity of respectively 6.9 Ah (Ampère-hours) and 11.8 Ah. 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 roughly 11 to 13 hours. (Roughly because charging the last few percent of a battery takes rather long) Using the included 12 Watt (2.4 A at 5 V) charger reduces this to 5 hours.