A restaurant was an establishment dedicated to serving food to customers. They could vary greatly in terms of service, food provided, and location. Someone who works is in the business of running restaurants is called a restaurateur. (DS9: "Nor the Battle to the Strong")
Earth was also home to Sisko's Creole Kitchen in New Orleans, Louisiana. This restaurant was owned and operated by Joseph Sisko, and focused on traditional Creole dishes such as gumbo. (DS9: "Homefront", "Paradise Lost", "Image in the Sand")
Space stations Edit
With a constant source of customers, space stations were ideal places for restaurants.
Deep Space Station K-7 contained a restaurant. Since it was a Federation station, the restaurant did not serve raktajinos. Cyrano Jones was known to do business in this restaurant. (TOS: "The Trouble with Tribbles"; TAS: "More Tribbles, More Troubles"; DS9: "Trials and Tribble-ations")
One of the largest establishments on Deep Space 9 was Quark's, which featured gambling tables (including Dabo tables complete with Dabo girls) and a bar along with normal restaurant functions. (DS9, et al.)