Most states and some localities have a sales & use tax. The sales tax is collected by the seller at the time of sale and then remitted to the state.

For internet sales that do not collect a sales tax the State/Locality then has a plan B for collecting $$ called the "Use Tax". Usually the sales tax rate and the use tax rate are exactly the same. In a nutshell, the Use Tax is a tax for the privilege of using something you purchased that you didn't pay any sales tax on.

For the most part the Use Tax is an honor system tax. The buyer purchases an item online, mail order, or QVC, etc... and doesn't pay any sales tax on it. The buyer is then required to self report to the state a Use Tax and remit the money to the state, this is usually accomplished on your annual income tax return. If the Buyer doesn't self report their Use Tax it's very difficult for the state to know a transaction took place, hence you get away with not paying any tax. If you're getting audited for some other reason the state may ask about any internet purchases you've made and then tack on a little something extra for not having remitted any Use Tax.