Some of it is pretty darn good, too!
I once helped hang a show consisting of Graffiti Art at the Cabrillo Arts Center in Santa Barbara. The Curator of the exhibition informed me of the following: Tagging, is when one person spray paints his mark, or graffiti, over another's mark, or graffiti. The sprayer is the "Tagger".
In some gang circles this is considered an act of war, and blood must be spilled.
In dark neighborhoods the graffiti lasts a longer time, but eventually it gets whitewashed over; destroyed, like a Tibetan Sand Painting...
Nothing lasts forever.