[Zubersoft]

It's a scalable vector graphics file that you can create in applications such as Inkscape (which is free). The drawing instructions are stored in an xml-based file, so instead of the application reading a bunch of pixel data and drawing that to the screen, it reads the drawing instructions and follows those to recreate the image that was stored in the .svg file. That is why it scales properly to any size - the drawing command instructions are just scaled instead of the lossy approach of scaling pixels in an image. I know that got a bit technical, but I'm not sure how else to explain it.

Mike