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 use cardboard, paper or other types of media (such as plastics) to produced templated graphics and text. You merely cut out the shapes from your paper/cardboard/other surface, place said surface onto a wall (example), then spray or roll-on paint.
In essence, the method of stencilling involves applying paint across a stencil to form an image on a surface underneath. Sometimes multiple layers of stencils are applied on the same picture to add colours or give the image some depth.
There are myriad reasons for being a stencil graffiti artist. For a few, it is an easy method to put across a political point. Other stencil artists value the PR that their artwork can acquire. Some simply want their art to be viewed. As the stencil stays unchanged throughout its use, it is easier for a stencil artist to speedily duplicate what could be a complicated artwork at a very quick pace, when equated to other prescribed tagging methods, so without question, there is also a compelling self-promoting reason to choose stencilling.
Banksy has used stencilling as a means to put a point across, often quite succinctly, providing the spectator to work out that very point in their head. This is oftentimes why people enjoy his work so much, as they force the viewer to meditate on the significance behind the artwork. Banksy art is now being reproduced on t-shirts, canvas and posters, such is the popularity of Banksy.