What is JPG?
JPEG is the format photographs live in. Its compression is lossy - it discards detail the eye is bad at noticing, and discards a little more every time the file is re-saved - which buys enormous size savings on photographic images and makes a mess of screenshots, logos and text. It has no transparency.
What is PPM?
PPM is a Netpbm image: a tiny text header giving the dimensions, then raw pixel values with no compression whatsoever. Almost nothing displays it, which is rather the point - it is what image-processing code and academic tools emit when they want to be trivially readable.