A bartender (also barkeeper, barkeep, barman, barmaid, or tapster, among other names) is a person that serves beverages behind a bar in a bar, pub, tavern, or similar establishment. This typically includes alcoholic beverages of some kind, such as beer, wine and cocktails, as well as soft drinks or other non-alcoholic beverages. A bartender, in short, "tends the bar". A bartender may own the bar or may be an employee. The term barkeeper may carry a connotation of being the bar's owner.
Bartenders are most commonly employed on larger starships designed as much for the comfort of the crew as to be a military vessel, such as Galaxy-class Starships, where the crew lounge (famously called "Ten Forward" on the USS Enterprise-D) serves in much the same function as the establishments listed above - usually for the purposes of crew morale and for entertaining guests aboard the starship.
Typically starbases would have numerous bars, restaurants, and other establishments as part of their promenade - as such, there will likely be an abundance of bartenders aboard the station at any given moment.