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.
What is PBM?
One bit per pixel - black or white, nothing in between. The most minimal image format in common use, and still the right one for scanned documents, line art and anything headed into OCR or shape analysis. Photographs do not survive it.