US Sales Tax is a consumption tax on the sale of goods and services to the end consumer. Unlike VAT or GST, it is only levied on the final transaction with the customer, and there is no charging or reporting requirements for the companies in the supply chain. In most situations, the seller is held responsible for the calculation, collection and settlement of tax to the local state.
It is charged by State and local authorities and administered by the merchant who is responsible for charging a combined % tax. A merchant must charge sales tax if it has a 'nexus' in the State, which can be a physical presence of employees, property or inventory. This is similar to the permanent establishment concept.
Sales Tax was first introduced in West Virginia in 1921. It has since spread to 45 states in the US. The US is one of the very few countries in the world that still applies a final sales tax; most OECD countries now operate a full VAT or GST tax regime.