A misleading term coined by the music industry and MTV to trick gullible, angsty teenagers into a false sense of rebellion, either against "mainstream" music scenes, society, or at least their parents.
Counter-cultures can range from obscure and "underground" (also another industry trick word) to being popular and mainstream.
When a counter-culture becomes mainstream (such as grunge
in the 90's and most recently, emo
), it is still marketed as being against the mainstream although music acts of that genre might be flying off the charts. Of course this leads to the more elitist counter-culturists who follow a counter-culture just to impress their friends by following obscure, unpopular and/or cult-favorite music acts.