Question

what is the difference between a png and a jpg

Answer 1

just the file format....otherwise they are all images...pngs tend to be very large and have more quality if memory serves me ryt

Comments

png's can often be the same size or smaller than equivalent size jpg's but with better quality

Answer 2

PNG: http://en.wikipedia.org/wiki/Portable_Network_Graphics
stands for Portable Network Graphics and was designed as a replacement for GIF that does lossless compression which means that your image doesnt get blurry or worse quality as you compress it

JPEG or JPG stands for Joint Photographic Experts Group http://en.wikipedia.org/wiki/JPEG
it is a lossy compression format for images (the image might get distorted or blurry with higher levels of compression)

the main differences between PNG and JPG is that PNG is specifically designed for website use (non lossy compression/small filesizes) and also supports transparency which JPEG does not

JPEG was originally designed for compressing photographs for easy storage/transmission (much like MP3 was for WAV files)

Comments

I have noticed a web page talking about the PNG format:
http://www.yiigo.com/support/kb/formats/png/
I hope it helps.