What is JPG?

JPEG is where photographs live. Its compression is lossy - it throws away detail the eye is bad at noticing, and throws away a bit more every time you re-save - which buys huge size savings on photos and makes a mess of screenshots and text. There's no transparency in it at all.

What is PPM?

Uncompressed RGB behind a four-line text header - about as simple as an image format can be while still being one. Nobody ships PPM to users; it exists as the thing scripts and teaching code write when they'd rather not link an image library.