Every retailer in the state of Texas is required to collect sales tax. Certain items are tax-free, such as bread or milk, but nearly everything else we need on a daily basis is taxed at the time of purchase.
The sales tax we pay is comprised of a state sales tax rate and various city and local sales and use taxes. Dallas and Fort Worth both support mass transit systems and they collect taxes from participating cities. Most cities also collect sales taxes.
The state of Texas provides an online Sales Tax Rate Calculator to help determine your local rate. To use the rate calculator, just type in the name of the city where the store is located (not necessarily the same city where you live). County name is optional in most cases.