Abbreviation for Quick Service Restaurant, the modern PC way to say Fast Food Restaurant. Instead of being seated at a table, placing your order with the waitstaff, and the food is brought to you on dishes -- a QSR has a drive-thru or you place your order with a cashier at the front counter and take the food in a bag or similar disposable carrier. The food is mostly pre-packaged and reheated, so you get your food faster than if it were made to order. "Fast food" carries a negative connotation as unhealthy, so the industry uses QSR instead to avoid the stigma and to account for the increasingly high-priced "premium" fast food options.
QSRs: Taco Bell, McDonald's, Burger King, Qdoba, KFC, A&W, Long John Silver's, Arby's, Wendy's, Chester's, Popeye's, etc.