Bandwidth or monthly traffic is the amount of data your site has displaced, directly from the hosting server. Included in this fee are: FTP transfers (website update protocol), downloading files from your site by visitors and emails sent / received (redirection and POP).

Usually traffic values are measured in bytes (megabytes, gigabytes, etc.). At where:

MB - Megabytes
GB - Gigabytes

Every time a person visits a page on a website, the amount of data on that page (html, images, flash, etc.) that is transferred from the server to the visitor's computer counts as traffic. For example, we assume a page in a site with a size of 10 KB, including the actual page in html plus what it contains (text, images, flash, etc.). Every time someone enters this page, 10 KB will be counted in the traffic in your account.