SVG, or scalable vector graphics, is a file format for storing vector graphics. This is a relatively new format standardized by the world wide web consortium, the same group that standardizes things like HTML. SVG files can be used for illustrations in books, magazines, and articles, and are also used for other things like scalable icons.
On Windows, there are a few softwares that are able to manage those files, like Corel Paint Shop Pro and I hope to see a support by StylePix too.