Stencil graffiti, made famous by Banksy in the last 4 or 5 years, has actually been a form of street art for 25 years. You can apply cardboard, paper or other types of surfaces (such as plastics) to make templated imagery and text. You simply cut out the design from your paper/cardboard/other surface, put then set this surface against a wall (for example), and roll-on paint or spray into the cut-out shapes.
In effect, the procedure of stencilling involves applying paint across a stencil to form an outline on a surface below. It’s also possible to employ multiple layers of stencils to produce an illusion of depth or even add colours.
There are innumerable reasons for being a stencil graffiti artist. For a few, it is an easy method to put across a political message. Other stencil artists appreciate the publicity that their art can obtain. Some only want their art to be seen. Because the stencil stays unchanged throughout its use, it is far easier for an artist to rapidly repeat what could be an intricated artwork at a very speedy pace, when equated to other traditional tagging methods, so without question, there is also a persuasive promotional reason to get involved with stencilling.
Banksy has used stencilling as a method to make a point, often rather succinctly, leaving the witness to get the message themselves. This is often why people love his artworks so much, as they make the viewer think. Banksy art is now being reproduced as posters, t-shirts and graffiti art on canvas, such is the popularity of Banksy.