Traditional graffiti is made by free-hand aerosol paints, but there are many other ways to do it. The use of the word has evolved to include any graphics applied to surfaces in a manner that constitutes vandalism. Graffiti is an art form, which is characterized by vying competition on the one hand, but also by high respect for the work of others on the other hand.
What can one say that has not already been said by another. There are a lot of scientific papers about that as well as a lot of internet chatter. The best will be to flatter yourself an own opinion.