Synthehol (a mash-up of the words of "synthesized" and "alcohol") is a man-made chemical variant of alcohol. It has the same taste and smell as "real" alcohol to most humanoids, but with none of the detrimental effects.
Served on starships at least as early as the 2250s, synthehol didn't become popular in the Federation until the 24th century. It became a regular ingredient in the drinks served on Starfleet vessels, as well as starbases.
Some bartenders, however, were known to keep a supply of genuine alcoholic beverages for special customers and occasions.
Most humanoids have an enzyme which breaks down the alcohol-like compounds in synthehol, thus allowing them to be free of intoxicating effects.