DISCLAIMER: Everybody knows that by definition graffiti is any illegal drawing. When we say "graffiti" here we are talking about the art style. More specifically we are talking about the art style that originated in the New York graffiti culture, around 1970. A "writer" is someone who does actual graffiti and more concerned with getting up (tagging or piecing their name on public property) .. A "graffiti artist" is someone more concerned with the style.
The second point is that art has no real rules. If you can rock a unique style, more power to you. Yet this culture has deep traditions which evolved in complexity and can weed out a "street artist"
The guides on this site are an attempt to help study some of the fundamentals found in graffiti art.
BASIC LETTER PRINCIPLES: THE 10,000 FT VIEW