USB power supplies (chargers) are available with different current ratings. From 1 amp per socket to 2.5 amp. Most of these are 5 volts (the standard for USB devices).
Thus a 2 amp USB socket, at 5 volts will deliver 10 watts of power (2A x 5V = 10W)
The I pad uses a 10 watt charger as standard. While mobile phones tend to use c 5 watt.
However if a mobile phone was charged using a 10 watt supply it would charge more quickly and not be detrimental to the battery.
Similarly an I Pad would be best charging in a 2A, 5V USB but could also charge in a 1A, 5V USB but would take longer to charge.
With regard to USB charging of laptops or computers this is different story.
Laptops need around 3A, 20V to give 60W – this is not available through USB but could become available over time with next generation USB type C developments.