Graffiti is writing or drawings, scribbled, scratched or sprayed on walls or other objects. Tagging is a form of graffiti where people put their own nicknames or symbols (tags). If it is done without permission on someone elses property it is a form of vandalism. Graffiti is particularly obvious on walls around the town and on public transport like buses and trains.
The official term used by the police for this type of graffiti or vandalism is criminal damage. Criminal damage is a serious offence and people committing it could be arrested and get a Penalty Notice Disposal (fine notice) a Reprimand or Warning, or they could face prosecution in court and up to 10 years in prison. The Anti Social Behaviour Act 2003 made it an offence for shopkeepers to sell aerosol paints to people under 16 years of age. Removing graffiti costs the UK over 1 billion a year.
Sometimes legal graffiti projects are arranged, because it can be very artistic and it can be used to make places look brighter and interesting. Graffiti and spray-can art is a form of street art and can be very skilful. But people should only get involved in creating graffiti if it is as part of an organised project or there is proper permission from the owner of the property (and not just where lots of other people have been doing it).
Most of the time graffiti on buildings and property does not look good and it is costly to remove. If it is left there, people can start to get bad feelings about living in an area, because it looks messy and people feel others dont care about their surroundings or about how other people feel. It might actually start to make people more frightened about crime in the area.