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?