Grafedia works by placing e-mail addresses or Web URLs in public places. When someone sends a message to an address, they receive a message or image back. Some grafedia initiatives are art projects, while others are using grafedia to provide information on historical locations and other points of interest. One project, Yellow Arrow, encourages people to place yellow arrow decals on public places (getting permission beforehand, of course) with a phone number through which others can get information. For instance, participants can place yellow arrows near a favorite restaurant and describe what makes that establishment so special.
Currently, grafedia is most prominent in large urban centers such as New York City and Toronto, which have a combination of pedestrian traffic and tech-savvy people. But if it catches on in other locations, it will be only a matter of time before everyone from marketers to urban planners latch on to it, using it as a tool to provide information in open spaces.
Source: AP (Yahoo!)