Bandwidth explained
Bandwidth is the amount of data transfer that you are allowed to have for a hosting package i.e. lets say you have a web page that is 48KB in size, now each time a user opens your site and goes to that page S/he downloads 48 KB of information on to his computer. If a 100 users view that page 100 times then there has been a total of 4800 KB of data transferred from the server on which your files are stored for the computer of users.

Size of web Page: 48 KB
Number of users who viewed the web page: 100
Total amount of data transferred: 48KB * 100 = 4800 KB

Bandwidth is the term that is used to denote the amount of data that has been transferred from your web space.