Just what it sounds like -- a bar where you go to breath oxygen. While the ingredients of the 'oxygen' are rarely listed, there's a lot of O2 involved, and no carbon monoxide or other pollutants. Nearly all oxygen bars will offer aromatherapy blends in their oxygen.
The various (advertised) benefits of oxygen are:
Combats stress and anxiety
Aids recovery from physical exertion
Aids in long life
Helps the body oxidize and eliminate the build-up of toxins and poisons in the bloodstream and body tissues
The aromas added to the oxygen treatments have numerous added benefits.
So what does it cost? Most bars in America charge something like $10.00 for 10 minutes, with a sliding scale for longer periods of time (the more O2 you buy, the less you pay, per molecule).