Actually two words, or hyphenated when used as a job title [i.e. bar-back, or bar backing]. A bar-back or runner, as they are commonly known in Europe, is a bartender's assistant. Bar-backs work in nightclubs, bars, restaurants and catering halls. They stock the bar with liquor, ice, glassware, beer, garnishes, and so on, and usually receive a portion of the bartender's tips. At high volume bars, the tips are divided where more than one bar-back is present.
They are often under the tutelage of bartenders and work their way into the job. They are there to simplify a bartender's job; bussing tables and dishwashing are common duties. In some establishments, bar-backs may also be responsible for the safety of the bartender.
While the legal drinking age in the United States is 21, the minimum age to work as a bartender or bar-back varies from 18 to 21.