Bandwidth

Bandwidth is an important thing you have to look in to when you are buying web hosting for your web site.
Normally bandwidth is defined as, 1.In a given time period the amount of data passed through a connection is measured in bits per seconds (bps).
1 kbps one kilobit per second - 1,000 bits per second
1 mbps one megabit per second - 1,000 kbps one million bits per second
1 gbps one gigabit per second - 1,000 mbps - one billion bits per second.

2. For web sites the file transfer is measured in GB/month. The web hosting server allows you to store bjects in MB size. The transfer of files uploading or downloading will be calculated in GB. Some time if you pass your bandwidth limit of a particular month your web site will not be accessible. Instead of web pages your site will display a note saying that your web site has passed over the month's bandwidth.

In that case you have to buy some bandwidth from your web host to last till to the next month. Normally 3$/GB for extra bandwidth they charge.

Here is a guide for calculating your needed bandwidth amount.
Say your average web page size is 50kb and you have visitors viewing 2,000 per day. So you will need a 3GB/month hosting minimum. So to keep it sure working if has more visitors buy a 4 to 5GB hosting.
If you are going to start a new web site go for the minimum GB limit and later you can increase it after according to the need. Most of the hosts provide more bandwidth as per the hosting MB size.