>>584788How does I Wikipedia/
Dictionary.com/Google/etc....
Anyway, a raster image (jpg/png/bmp/etc) is stored as color data for pixels. When you upscale it, you're trying to make new pixels from nothing. the stretching causes blurriness/blockiness. A vector image is stored as mathmatic data. A set of bezier curves and points along with the color data. Since the data is mathmatic, upscaling it doesn't decrease the quality at all. A vector the size of a stamp could be blown up to billboard size with no quality loss.
More info (scroll past the tuts):
http://vectortea.com/vtut/