Data transfer can be defined as the total amount of data (in bytes, kilobytes, megabytes, etc.) transferred as a result of communication to your website by both FTP, as well as data delivered to site visitors.
The data, in data transfer, consists of all elements that are transferred to the browser of any person visiting your website. Thus, all the html pages, images, audio/video content and other files that are delivered to a visitor from your site contribute to the data transfer for your website - every time somebody visits your site, they are taking up a fraction of your data transfer quota. Also, all data that you upload to your website as and when you make any addition/modification to it, also counts toward your data transfer.