Data transfer (sometimes loosely referred to as "traffic" or "bandwidth") is the amount of bytes transferred from your site to visitors when they browse your site.

[B]Don't believe any commercial web host that advertises "unlimited bandwidth". The host has to pay for the bandwidth, and if you consume a lot of it, they will not silently bear your costs. Many high bandwidth websites have found this out the hard way when they suddenly receive an exhorbitant bill for having "exceeded" the "unlimited bandwidth".[/B] Always look for details on how much traffic the package allows. I personally always stay clear of any host that advertises "unlimited transfer", even if the exact amount is specified somewhere else (sometimes buried in their policy statements). Usually you will find that they redefine "unlimited" to be limited in some way.

To give you a rough idea of the typical traffic requirements of a website, most new sites that are not software archives or the like use less than 3GB of bandwidth per month. Your traffic requirements will grow over time, as your site becomes more well-known (and well-linked), so you will need to also check their policy for overages: is there a published charge per GB over the allowed bandwidth? Is the charge made according to actual usage or are you expected to pre-pay for a potential overage? It is better not to go for hosts that expect you to prepay for overages, since it is very hard to forsee when your site will exceed its bandwidth and by how much.