I explain this:
Ok if you have 20 gigs of bandwidth per month. That means you are allowed to have 20 gigs of your website showed to people. If someone downloads a 20 gig file from your website, you have used your limit. But if you just have a simply website with text and images, you can get 1000's of hits before it is used up.
If you have a 10kb page loaded 10000 times that means you used up 100,000kb b/w or 100mb which is only 1/10th of a GB