SVG is a vector format. PNG is a raster format.
Vector images only store paths and stroke details, which are later drawn by the image viewer. This allows vector images to be scaled up as large as you want without pixulation; the paths will just be re-drawn when you view it at any size.
Raster images are saved pixel-by-pixel. This means that if you scale a raster image up, it will have blurs and pixulation.
Vectors are used for different purposes than raster images are. For example, logos are often created as vectors so that they can be scaled to fit whatever designs you have in mind for it, from an icon to a tshirt to a huge banner. In vector image editing programs such as Inkscape and Adobe Illustrator, you can draw vector paths and then have it fill in the inside with a color or gradient. This scales as well since it's a vector, contrasted by raster painting which looks like shit when you zoom it up.
Pony templates properly saved as vectors give the advantage of being able to instantly fill in a skin color of your choosing, vice raster templates which often look choppy on the inner borders and can require much touch-up to even look decent.