A bar is an establishment where alcoholic beverages are sold to be drunk on premises. It can be either an independent business or a section of a restaurant or hotel.
Alternatively, "bar" can refer to the specialized counter on which the drinks are served, and it is from this term that the establishment itself as a whole gets its name. Also, the "back bar" is a (sometimes ornate) set of shelves of glasses and bottles behind that counter.
In the UK 'bar' is usually short for 'Wine Bar' where as the name suggests mainly wine is sold and drunk.
Bars range from down-and-dirty "dives," little more than a dark room with a counter and some bottles of liquor, to places of entertainment and the elegant watering holes of the elite.