What is the difference between USB charging and AC charging? I've recently got hold on an HTC One X  having moved from an HTC Desire.

I noticed that my One X charges very slowly when connected to the USB socket on my PC, whereas the Desire was comparatively fast. When charging the One X from the supplied AC adapter the charge is very fast. So, I purchased a new Mains Adapter to keep in the office.

However, when connecting the phone to this mains adapter the device still says Charging : USB under the battery status tab and it isn’t as fast as I thought it should be

So, is there some difference between the supplied AC adapter and this third-party one I have purchased? Presumably the power supplied s not as high or is there something more?
In fact the charging time depends on two points: 


the output current of the adapter or the USB. The output current of the PC is only 0.5a, it may charge slowly than most of the AC adapters (most of the AC Adapters are 1A or more).
the input current and the charging convert rate of the phone. Different phones have different input current due to Limited current protection for them. For example, the limites current for Nokia is 0.5a, the input current is only 0.5a even you use an ac adapter with 1.0a output. Beside, the charging convert rate is also different according to the models.