One who works behind the bar. This could apply to a barkeep serving alcoholic beverages; it is also often used to describe espresso workers at cafes.

A 'professional server.' Commonly used as the job name for the coffee jockey who whips up your double grande hazlenut latte, no foam. Should be a master of crafting fine espresso beverages, but more often than not is a disgruntled teenager who doesn't clean the equipment very thoroughly. or often.