>>6413872JPG uses lossy compression while PNG doesn't. Basically PNG is larger in file size but the quality is that of the original image. It's most noticable with small text, it's what causes all that weird static shit in the background on all those crappy screenshots from tumblr or whatever else you see around here.
See this for example -->
https://upload.wikimedia.org/wikipedia/commons/7/72/Jpeg-text-artifacts.gif 
