Yes, it's all 5 volts DC, per the USB spec.

The only difference is that different versions of the USB spec support different power levels out of that USB port. The standard limit in USB 2.0 is 500 mA, and 900 mA in USB 3.0. Some modern smartphones and devices (e.g., Kindle Fire) need closer to 1000 mA. The iPad needs 2000 mA, which is not in line with the USB spec, but Apple does what it wants.

The device you plug in will signal to the charger or PC USB controller how much power it wants. If you plug in a device (e.g., an iPad) to a charger that can't provide enough power, the device will either charge more slowly or give a warning that it can't charge from the connected power supply.

If you plug in a device that needs less power than the connected supply can provide, the device will charge normally and draw only the power it needs.

In short, feel free to mix and match USB power supplies and devices. You won't break anything. At worst, your device may not charge from the connected supply.