Bandwidth & data transfer to a large degree are somewhat synonymous. In a nutshell, it refers to the amount of information transferred to and from your web site by you and your visitors. This boils down to 3 major factors: 1. The amount of traffic you receive 2. The size of your site 3. The types of files offered on your site.

Very simply put: If your web site is 2MB including pages and images, and 1,000 people viewed every page on your site you would transfer 2,000MB.