The iPhone charger comes in 2 parts the plug and a cable. This is done to make easy charging and data transfer through USB connection. Below is a list of the different outputs from the different devices
As you can see the charge gets better using the more powerful charging device the USB being the slowest charge and the iPad plug being the fastest.
The first of the cable was this type with a larger charging point on the iPhone.
But as Apple are always looking so save space they managed to shrink the charging point down to a iPhone 5 charging cable.
Obviously the next move will be wireless charging with no power point.