I've only heard bars referring to Xanax. It is commonly prescribed for anxiety disorders, but people generally take it for recreational purposes. The 2 mg tablets are bar-shaped and can get broken into smaller doses.
Bars is the slang term used for two different drugs. The first is marijuana; the second, is the prescription drug Xanax, which is used to treat anxiety and depression. Xanax is more frequently called Bars than marijuana.
It is just a street name for the drug Xanax, a common anti-depressant drug. The name reflects the bar shape of the pill itself. They are usually 2mg tablets.
Bars usually refers to Xanax(which is a brand name for alprazolam)pills. They are benzodiazapines which puts them in the same class as Valium and Lorazepam,