What is PBM?
PBM is Netpbm's bilevel format - one bit per pixel, pure black or pure white, nothing in between. It is used for scanned documents, fax-era imagery and dithered art, and its packing convention surprises people: a 1 bit means black.
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.