Street art is art created in public, urban, spaces. The whole notion comes with an unfair perception of unsanctioned art, and while there is a fair amount of street which is illegal or unsolicited, the opposite is more and more the case with sponsors such as local governments and owners of buildings.
The umbrella term has come to include traditional graffiti artwork, stencilling, video and light projections, sculpture, sticker art, street posters, art intervention, pavement art and a host of other things.
For more on this topic, please consider reading these articles:
Urban art has earned ist art gallery and street credit
The Street Art Project