Bandwidth is defined as a range within a band of frequencies or wavelengths. In computer networks, bandwidth is often used as a synonym for data transfer rate - the amount of data that can be carried from one point to another in a fixed amount of time. However, there is quite a difference between these two terms technically.
A measurement of the data transfer rate over a medium (network). Your site will load faster if you have more bandwidth.
Data Transfer :
The traffic generated (or amount of data transferred) from a site. The amount of data transferred will depend on the number of visitors to the site and the file sizes of web pages (including multimedia files).