You are destroying property values.
It is vandalism, though writers consider it art (well, usually - not so much gangs!). So, if you're doing some graf on something that doesn't belong to you, it's bad. Go crazy on your own property.
It's punished by a fine and/or community service, and if you're an adult, possibly jail time. It all depends on where you're located. Most of the time you'll get a fine and community service, I think, because they want to make sure you've learned, and it does cost money to repair damage.