It is very common to find the two words - bandwidth and data transfer, used synonymously in the web host's pricing plans. However, technically speaking, there is quite a difference between these two terms
Bandwidth refers to the total amount of data that can theoretically be transferred at any one time. Data Transfer refers to the amount of data that has actually been sent from the Server to the client or vice versa. An analogy would be a road, the number of lanes on a road would represent Bandwidth, the number of cars that travel along that road over a given period of time would represent Data Transfer.
Everytime a user opens a page of your website the data is transferred from the Server to the user i.e. the users web Browser . Typically, a page with a few graphics (GIFs or JPEGs) will probably require the transfer of about 50KB of data to your Browser .
If the bandwidth is high, the website will load faster because it can transfer more data per second. This does not mean that the data transfer will be high, because the amount of data transferred will depend on the number of visitors to your website and the file sizes of your website pages (including image, audio and video files etc.). The greater the number of visitors, the greater the amount of data transferred from your website, and so larger will be the data transfer
Bandwidth is usually expressed in bits per second (bps) or bytes per second. It is a very important measure of an input output (I/O) device. For example the modem that we use to connect to the internet has a certain maximum bandwidth (say 56 Kbps). On the other side a cable modem can transfer more data per second and usually has a bandwidth of around 200 - 1000 Kbps. A T1 line to the internet can transfer at the rate of 1.5 Mbps or higher. Data transfer is measured in bytes, kilo bytes (KB), mega bytes (MB) or giga bytes (GB). If a website delivers 100,000 pages each month with average page file size of 20 KB, the data transfer per month will be 2 GB. Almost 80% of websites on the internet uses less than 5 GB of data transfer per month.
30 Gigabytes of monthly bandwidth or data transfer translates to about 30,000 unique visitors per day. The average website uses less than 250MB of bandwidth per month and receives about 50 visitors per day, so only very popular sites ever exceed 30GB.