A png (Portable Network Graphics) file is a raster or bitmap image file format. A raster image is made up of a fixed number of  pixels [or building blocks] that form a complete image. 

The image cannot be enlarged without distortion occurring. (When you zoom in on a raster image, the pixels become visibly grainy.) Common raster image files include png, jpg and gif formats. 

A svg (Scalable Vector Graphics) file is a vector image file format. A vector image uses geometric forms such as points, lines, curves and shapes (polygons) to represent different parts of the image as discrete objects. These forms can be individually edited. A vector image remains crisp and clear at any resolution or size.