Although people often use these terms interchangeably, there is a slight difference. Data transfer is all of the information transferred from your web site to your visitor's computer; bandwidth is the capacity for this transfer.

You can visualize data transfer as the number of cars on a highway, and bandwidth the number of lanes. While bandwidth remains fixed, your data transfer will ebb and flow, depending on how many visitors you have, how many people are using your site, and how complex your site is.