Difference between USB Data Tethering and 3G Mobile AP?

Hi there,
My cell plan has 500 MB data plan, but when I tried USB tethering last month to just check it out, I got charged even though I did not use out of plan limit of available limit. Now my question is, if I make my 3G internet (again with 500 MB limit) as a wifi via Network Settings > Mobile AP > Add AP, will I just make my phone as a wifi router for other devices and NOT make it tether like USB tether?

Is there a difference between USB tethering and using mobile's AP (Access Point) to share internet via wifi from available 3G network?

I have Froyo 2.2.1 Canadian version. Thank you very much.