What is SVG?
SVG is a drawing rather than a grid of pixels: shapes, paths and text described in XML, so it stays sharp at any size. That makes it right for logos and icons and useless for photographs. Converting it to a raster format fixes the size it will ever be.
What is PNG?
PNG is the web's lossless image format: compression that never degrades the picture, and a real alpha channel for transparency. It is the right choice for screenshots, logos, line art and anything with sharp edges or text.