Bandwidth is the amount of data transferred from your account at the web hosting server to visitors of your web site. It is, therefore, also known as data transfer. Please note that visitors to your web site not only refer to fellow human web surfers but also search engine bots (programs sent out by search engines to index web sites). In fact, the data transfer by you using FTP (for web site maintenance purposes) is also counted in your bandwidth.
Web hosting companies generally mention a per day or, more commonly, a per month bandwidth in their hosting packages.
It is difficult to predict the bandwidth requirements for a web site because there are many variables involved.
Any way, let us look at some figures to gauge the requirements of an average web site. Consider a web site with 200 web pages where each page is 100KB in size (including images and other external files). If this web site gets 400 visitors each day and each visitor views 3 pages on an average, the web site would be transferring 400 (visitors) X 3 (number of pages viewed by each) X 100KB (file size of web page) = 120,000KB or 120MB each day or 3.6GB per month.
We would all like to believe that your web sites would be the successes of the century but to dampen your “vaulting ambition”, it is unlikely that a new web site would get 400 visitors, unless you start with a lot of funds for online advertising like Pay-Per-Click ads. But if you have so much moolah to begin, you would not be bothered about the amount of data transfer anyway.
Coming back to the discussion, you can only get a correct estimate of your web site bandwidth requirements once you put it online.
In any case, the data transfer offered by most web hosts would be more than sufficient for you to start operations and if your web site crosses the set data transfer limit, you can always purchase additional bandwidth.
There are several web hosting companies offering unlimited bandwidth. However, there is more to it than what reaches the eye. Based on their experience, such web hosting companies know it is unlikely that web sites hosted on their servers get high levels of data transfer. Hence, they go ahead and freely advertise unlimited bandwidth. I would recommend that you read the small print and confirm this with the web host. But like I said above, the bandwidth requirements of a new site would be negligible compared to the data transfer offered by web hosts. For example, at the time of writing Lunarpages web hosting company was offering 2500GB monthly transfer.
If you want more information, you can read about unlimited bandwidth and where to get it (?) in a separate article.