Wikipedia describes yarn bombing as "a type of graffiti or street art that employs colorful displays of knitted or crocheted cloth rather than paint or chalk." Knitted, crocheted or woven pieces are sewn around inanimate public objects such as bike racks, light poles, telephone boxes and abandoned buses. The purpose is to bring some beauty and wonder into otherwise drab and everyday surroundings. It is less damaging than painted graffiti and costs next to nothing (a pair of scissors and a few minutes work) to be cleaned up and taken away. Some find it to be pointless and no better than any other form of vandalism, but it can be easily argued that it does cause people to stop and take a fresh look at their surroundings. Larger groups such as Knitta Please and Knit the City often do large scale projects, but smaller tags can be easily created and installed by lone artists. This instructable will take you through the basic steps of yarn bombing.
For more information, check http://www.yarnbombing.com
, a blog devoted to yarn bombing. There is also a newly published book on the subject called Yarn Bombing: The Art of Crochet and Knit Graffiti by Arsenal Pulp Press.