What a picture style is, is a bundle of settings that yield a certain type of processing when JPEG conversion is done. In effect, for the JPEG shooter, selecting a picture style is like selecting a maker and type of film for a film camera.
The camera comes with a set of predefined styles, and you can also create your own using Canon's "picture style editor", which you run on a computer outside of the camera.Tom's Digital Photography Info / tom@mmto.org