Stenciling (or stencil art, technically, is the use of a stencil to create a work of art, most often with paint. Stenciling can also be connotated to mean a method of quick street art (a form of tagging or graffiti esentially). In this case the stencil makes a cleaner, crisper image in a shortened amount of time so that apprehension is less likely. The same stencil can often be used multiple times, making the process of repeating a viewpoint, or reposting a peice of art simpler and more efficent.
Tips
Be sure that your stencil is securely fixed to the surface that you are painting
Secure your stencil (perhaps with repositionable adhesive) to keep your image clear
When using spray paint, use a resporator
Use plastic (acitate preferably) for a reusable, durable stencil
Only stencil what you own