Some ISPs put monthly data transfer caps on personal web sites to prevent users from bogging down servers with very popular websites. They can either bill you extra when you go over the cap, or actually disable the site for the remainder of the billing cycle. Bandwidth would be the rate at which your content is delivered. And "data transfer" in this case would be the amount you are allowed to serve in a given period. If you have a graphics-intensive or flash site for instance, you might want to be eyeing that "data transfer" limit.