In more detail
Unlike VAT, it is applied only at the final sale to the consumer; businesses buying goods for resale usually present an exemption certificate instead. In the US there is no federal sales tax: rates are set by states and often also by counties and cities, so the combined rate depends on the exact location, and a few states have none. Some items, such as groceries or prescription drugs, are exempt or taxed at a lower rate in many places. Buyers who pay no sales tax on an out-of-state purchase may owe an equivalent "use tax".
Why it matters
Shelf prices in the US usually exclude sales tax, so the amount at the till is higher than the label. To find the pre-tax price from a receipt total, divide by 1 + rate; multiplying the total by the rate overstates the tax.
Example
At a combined rate of 8.25%, an $80 item costs 80 × 0.0825 = $6.60 in tax, $86.60 in all. Going back from the total: 86.60 / 1.0825 = $80.00. Taking 8.25% of $86.60 instead would give $7.14, which is wrong.
Common confusion
Sales tax is collected once, at the final sale; VAT is collected at every stage, with businesses reclaiming the VAT they paid.