PNG files are a single frame. GIF files are a compilation of multiple frames that are, as Knox said, cycled through to create the animation. GIFs can contain as many PNG files as you want; however, this may not necessarily be the case as you can use any image type in the creation of a GIF. PNG files maintain 100% of image quality which is superior to JPGs as JPGs lose image quality due to compression, resulting in a grainy image. JPGs can be exported as PNG files but this will not revert the compression. The final file format of an animation in this case will always be a GIF. This is how it is, this is how it always has been.