Things used to be easy and unambiguous. You found advertisements in magazines and on billboards, art in galleries and museums, and people expressing themselves by means of graffiti and defacing of private/public property was thought of as a nuisance by most. This is an oversimplification, but bear with me.
Over the years, as the urban environment got denser, it also got more cluttered with ads, and street art took on a new poignancy and presence. Today, anyone in an urban environment is subjected to thousands of commercial advertisements, but also non-commercial messages per day.