What did habermas mean by the "public sphere"?

What did Habermas mean by the Public Sphere? Is this concept at all useful for understanding the role of the mass media in contemporary society?
Jurgen Habermas by many is considered the most influential thinker in Germany; his ideas of the "public sphere" were developed in the enlightenment era of the eighteenth century and were considered revolutionary. These ideas in some parts of the world are still highly regarded and have become an integral part of the way in which some societies understand themselves.
Habermas' primary goal was to develop a theory of communication rooted in a conc…