An example of graffiti on Deep Space 9
Graffiti on the left of a metal box
Graffiti on glass behind Spock and Kirk
Graffiti was an image or images placed on a surface in public view. This was usually done without permission with the intent to deface the surface affected. Such activity was considered a crime in many jurisdictions, including Deep Space 9. (DS9: "Crossfire")
Graffiti was often used to convey a political message. Members of the Alliance for Global Unity did so by spraying the symbol of their group on Deep Space 9 in 2370. Their intent was to let both the Bajoran and Starfleet crew know of their presence. It was seen in at least two places on the station, including the door of Benjamin Sisko's crew quarters. (DS9: "The Homecoming")
Ojuy Gel was arrested in 2372 for spraying graffiti to protest the visit of First Minister Shakaar Edon to the station. Afterward, he was sentenced to three weeks of community service for his crime. (DS9: "Crossfire")
Another example of graffiti can been seen when Sulu, Scotty, and Dr. McCoy examined an advertisement while trying to figure out how to make a whale tank while visiting San Francisco in 1986. Graffiti was visible on a metallic box nearby. More was seen on glass behind Kirk and Spock on a bus. These graffiti markings were popular in the 20th century. (Star Trek IV: The Voyage Home)