What is bandwidth?

Think of bandwidth as the freeway your data must travel to get to your computer. Because the freeway must be built to handle all the traffic (data) at rush hour, the bandwidth you use is determined by amount of traffic (visits) at your website. How is the bandwidth (monthly data transfer) used by your site determined?

This figure is an average. Bandwidth usage "samples" are taken every 5 minutes throughout the month. At the end of the month, the highest 5% of these samples are discarded. The highest remaining sample is then used to calculate bandwidth usage. (You may hear this referred to as the 95th percentile.) Putting a cap on the bandwidth still allows users to access the server, it just slows down the connection (so the users are on the freeway, but in heavy traffic).

Most hosts include a certain amount of bandwidth in their monthly charges. Anything beyond that, and there will be a charge.  The reason so many hosts shout "Unlimited disk space! Unlimited bandwidth!" is because they honestly don't expect the majority of users to use a huge amount of space or bandwidth.  Be sure to check that point when choosing an ISP.  One site shows the amount of bandwith it is using at any given moment - check out the Spedometer.