Sales tax, as you know, is collected at the time of a sale. However, when a company ships an item to a state in which they are not located, they do not always collect sales tax. The Amazon tax is the sales tax on purchases through Amazon. However, the tax, called a Use Tax, is paid by the purchaser, rather than the seller. Therefore, when you buy something from Amazon now, they will send you a report of your purchases each year and you are liable to pay the use tax to your state department of revenue. Use tax is paid when you purchase an item from a company and they do not collect sales tax on it. You, the consumer, pay the Use tax on the item which was shipped to you in the state in which you live.

Amazon.com has been criticized for collecting sales taxes from customers in only eight U.S. states. Several states have passed or are considering "Amazon tax" laws designed to compel Amazon to collect local sales and use taxes from customers. The U.S. has no federal sales tax.
There is a really good explanation with this link. Hope this helps
http://en.wikipedia.org/wiki/Amazon_tax