Survivors have the ability to spraypaint the walls of any block, leaving a message visible by any character, be they survivor, zombie, or dead body, who is on the block. Every type of block has suitable surfaces available, and buildings can be graffitied on the inside walls as well. The graffiti will appear as Somebody has spraypainted ... onto a wall, where ... is replaced by whatever text you enter; up to 50 characters. Spraypainting costs 1 Action Point (AP), unless one is spraypainting on a Billboard, in which case the cost is raised to 10 AP.
In order to spraypaint, a player must be carrying a Spray Can. For each message you leave, there is a chance that the spray can will run out. Attempting to spraypaint something vulgar will cause the spray can to to run dry before anything is even written, wasting both the item and the AP.
Painting over an existing message will overwrite that message with the new one. It is possible to improve the appearance of an existing message by spraying the identical words repeatedly; however, there is no known in-game benefit to doing so.
The Tagging skill gives experience for spraypainting certain types of blocks or buildings, and makes spray cans last longer.
Spraypainted messages can be defiled by zombies, rendering them barely readable until they are cleaned or spraypainted over by survivors. Messages on billboards, however, can never be defiled. This, combined with the high cost of spraypainting on billboards, tend to make those messages last longer.
There is also a Great Graffiti hall of fame where amusing, famous or otherwise memorable graffiti is immortalized.